Meralygos

 Abilities in Combat.

 How does a blue dragon fight? A collection of blue dragon abilities we have seen from games and other sources.

•  Spellmaw, Manaclaw, Scryer - The three drakes of Mazthoril. They all used a  Frost Breath and were immune to frost/arcane. Scryer had an  amplify magic debuff (increases all damage taken by 100%!), a frost breath and could see through stealth and invisibility, as the name implies.
•  Kaldrigos, the Dread Wyrm. Five heroes versus Kaldrigos! The mighty wyrm is defeated by five adventurers. A shadow priest was killed by  Arcane Breath. A dwarf warrior stood behind the dragon (Tail Swipe). The paladin had a sword that broke during the fight. He then fell upon it... And then we have a rogue who pierced the dragon’s heart with Silver Kris of Korl - that along with a debuff.
•  Azuregos. Frost breath, Cleave, Mass Teleport, Spell-Reflection, Tail Swipe and Mark of Frost (the latter freezes those that died, preventing them from ressing and reaching Azuregos). Also, see Manastorm. Immune to frost/arcane. Later on, in Cataclysm, Azuregos is met again fighting the numerous dragon hunters that the Twilight's Hammer sent to kill him. He flies above them, releasing  Ice Breath that freezes them in blocks of ice.
•  Tyrygosa, in the Sunwell trilogy comic, summons gold and radiates an explosion of magic which melts the undeads, uses breath attacks. Later, in the Shadows of Outland, she unleashes breath spells and spells that makes the demons she fights with explode. She is able to project an image of herself during the assault at the Dragon Soul. 
• Madrigosa. Fighting against Brutallus, she uses the following abilities: Freeze (Frost Blast), Frost Breath, Frostbolt. Also some sort of  Encapsulate, an arcane prison in which she encases Brutallus.
•  Arcanagos. Uses spell-reflect before being killed by Medivh.
•  Tarecgosa - She helps the player in the quest "Through a Glass, Darkly". There, she releases a powerful AoE Frost Breath attack as she flies above her enemies, entombing them in ice which kills them.
• Nexus Watcher, Crystal Wyrm. Blue dragon NPCs found in Northrend, guarding Coldarra and the Azure Front (respectively). The first uses netherbreath (arcane breath),  Intangible Presence. The second a regular frost breath. There is also the Azure Dragon NPCs found at Wyrmrest Temple, that use frost breath. All these three dragons do indeed fly. 
•  Cyanigosa in the Violet Hold also utilizes mass teleportation, forcing his enemy in front of her -   Arcane Vacuum. She also uses Blizzard, Tail Sweep, Arcane Breath and  Mana Destruction.
•  Ley-Guardian Eregos. Killed by drakes. During the encounter, he often fades from this plane of existance [Planar Shift] and becomes immune to magic. Then, he proceeds to summon Planar Anomalies to destroy the enemy drakes. He always flies during the encounter. During the battle, he releases arcane bolts and arcane bolts volley.
•  Sindragosa. In a similar way to Cyanigosa, who uses the spell mana destruction, Sindragosa utilizes  Unchained Magic - causing Instability. She also Freezes her enemies into a tomb of ice and then, soaring above them, destroys the enemies with Frost Bombs! Furthermore, she also uses: Frost Aura, Cleave,  Icy Grip (much like death grip, but targets the entire raid),  Blistering Cold (a charged explosion of frost magic),  Mystic Buffet, Tail Sweep and Frost Breath.
•  Drakos the Interrogator -  Magic Pull, teleports all player near himself. Also summons  Arcane Bombs that float around the area, exploding on occasion in a burst of arcane energy.
  
•  Varos Cloudstrider - Uses  Amplify Magic,  Energy Cores (a portion of the platform is energized with arcane, dealing damage to all that stand above it), and calls  Azure Ring Captains, drakes who use  Arcane Beams.

•  Scalesworn Elite. A blue drake that fights atop of the Nexus, in Coldarra. They guard the upper ring and have two abilities:  frost breath (frostbolt) and  arcane surge. The latter being very powerful. 

•  Kalecgos. In the Sunwell Plateau, Kalecgos will spam  Arcane Buffet, some sort of arcane explosion which deals stacking arcane damage. Frost breath. Tail lash.  Wild magic - a spell that affects the magic spells in an unpredictable way, ending as either a buff or a debuff. 
• Later on, against Kil'jaeden, Kalec will play the role of support. He shall channel magic through elven orbs, enchanting them in such a way that they may be used to take up the power of the blue dragonflight.The adventuerers that touch the orbs become blue drakes that release arcane breath (restoring mana or haste), blink and also make up a "shield of the blue", a powerful defensive ability. 
• In Azshara, Kalec helps the player once again. He teleports behind a black drake and kills him with a powerful "frostfire" breath. 
• Novels and stories. In the Sunwell Manga and later on in the Night of the Dragon book, Kalec can  summon a personal blade of his own, a trick he later teaches to Krasus. He is able to use many mage spells such as a slow fall. In the Shadows of Outland comic, Kalec also used an Arcane Surge against a nether dragon, killing him istantly from behind. In the Twilight of the Aspects, Kalec is seen as a talented conjurer, being able to summon an orcish chair, conjure beer, and even a table all by moving a single hand! He also uses frost breath attacks. Notably, Kalec can change (even before being an Aspect) between frost and arcane, depending on the narration.
•  Kalecgos the Spellweaver. Once more, against Deathwing, Kalecgos fights alongside mortals and aids them. There, he increases damage done by 20% and grants the buff  Spellweaving at the yell of "I will charge you with arcane energy to blast your foes!". Along with other dragons, he also enchants togheter Dragonwrath, binding the spirit of Tarecgosa to the essence of the Firelands the adventurer brought to him. In Legion, he uses frost breath multiple times.

•  Malygos the Spell-weaver. In-game, Malygos can be fought in the Eye of Eternity. He attacks with the usual  arcane breath. He exploits his massive wings to create a vortex and will also release an  Arcane Storm ability (some type of arcane barrage). After taking several damages, he begins to fly. He shall use a surge of power and  Power Overload will occur around the battlefield. He shall then use a charged arcane surge against the platform, destroying it. The third and last phase is a battle between Malygos and red drakes. Malygos will unleash once more his deep breath (Surge of Power, a powerful Arcane Surge) and will release a  Static Field of arcane magic. In the battle against Keristrasza, he does assault other dragons with frostbolts and a freeze spell.
• Novels and Stories. In DotA, he uses frost breath. In the WotA novel, Malygos used an elaborate version of a desintegration beam and a protective shield which looked like grey smoke. In the SoO volume II, Malygos absorbs the nether dragons by exuding several tongues of arcane from himself, which wrap around the dragons and absorb them within himself! DotD: he releases a  breath of cold, ice magic against Neltharion. After the frost breath damaged Neltharion, it transformed, turning into a thousand thousand tiny crablike vermin that sought to tear at the scales and flesh of their host. Later on, he releases a storm composed of "icicles of lightning" which attacked from all directions, damaging in form of intense heat and numbing cold alike! Later, he turns clouds to his will, turning them into chains which capture Deathwing's jaws.

•  Azurewing Drakes - Stellagosa, a friendly blue drake, assists the player against a fel-touched Warden. She uses a charged Arcane Breath (model an arcane barrage). Azurewing Drakes fight against the wretched nightborne. They released Arcane Breaths to keep them at bay. Later on, in a world quest, they release magical beams from their mouth to destroy the portals of the Burning Legion. Kharmeera is a brilliant enchanter.
• Senegos projects an  Astral Image of himself and later on helps in enchanting togheter several elements of different sources (nightmare, azsuna's eye, etc etc) during the questline to unlock a new legendary look for your artifact.

Breath Abilities.

A dragon's iconic spell. They affect, in a frontal cone, everything they come in contact with.

 Frost Breath - An iconic ability of blue dragons. How does it work? In-game, look is either that of an azure vapor, a frostbolt or sapphire flames - all having pretty much the same effect: inflict frost damage and reduce movement or attack speed. Type of harm: entombs in ice, freezes body temperature and may exhaust strenght, lowers agility, slows reflexes, causes certain body parts (those hit) to be frozen or become insensible, body becomes heavier and energies are depleted. Increases rates of reaction.

 Freeze or Frost bombs. Eventually, frost breath spells may also freeze and encase the enemy in a tomb of ice. When they do so, they usually don't come out in the shape of a cone, but rather in the form of a frost bomb (a concentrated orb of ice or a frostbolt). Some blue dragons can use bombs of ice, releasing huge explosions of frost magic.

 Arcane Breath - Much like the above, Arcane Breath is an ability conjured from within the dragons' mouth. Once released, it makes impact with sheer arcane might, usually striking in the semblance of arcane flames (violet shade). The Arcane Breath spell is somewhat unstable: it hits for a high amount of arcane damage and also leaves a debuff (Uncontrollable Energy of Cyanigosa) that damages over time the enemy. In some occasion, it may even burst into an explosion of arcane energy (Arcane Breath of Malygos). Such a debuff is indicative of arcane radiations still lingering on the targetted area, which are overcharged with magic. How the spell feels is not clear. However, the radiations are likely bond to damage everything they twisted, making this spell akin to fire as it leaves unstable energies of pure arcane that can melt skin, plate and even solid rock unless quickly subdued. Arcane breath can be concentrated in the form of an arcane surge (see below).

Breath: Revitalize. Arcane breath(s) can also grant benefits to what they touch, such as restoring mana and increasing spell haste. Such magic likely allows the allies to regain strenght as they are fatigued by the battle. It looks like a breeze, or a sapphire cloud.

Physical Attacks

 Tail Sweep - Dragons' tail culminate with an huge caudal bat, usually made of bones or an other tough material. The caudal mace is a powerful offensive weapon: the tendons of the tail aren’t much elastic, transmitting to the tail a great strength to be used as a powerful mace. In other words: it is unwise to stand behind a dragon!

Wing Buffet. Flapping wings to summon waves in the wind, likely disorienting or slowing down the enemies

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 Cleave - Claws of a dragon may take more than a single target, damaging all nearby enemies in a single hit.

Defensive Abilities:

 Phase Shift - Eregos uses the ability [Phase Shift], becomming immune to magic damages.

Tooltip says: "Shifts the caster to the astral plane for 18 sec. While shifted the caster is immune to magic damage but cannot move or attack." However in Warcraft there is no other mention to anything such as the Astral Plane and the RPG denies it. So, what does this spell do? A similar variant of this spell is also used by Nexus Watcher(s) with the name of [ Intangible Presence] - reduce damage taken. During Warlords of Draenor, a similar spell had been used by mages. It was called [Evanescence]: the caster fades into the nether, avoiding all attacks and still being able to cast and release spells.

So the Phase Shift basically makes the caster fade from this plane of existance, to some extent - not entirely. Doing so, allows him to connect with an arcane domain yet, while he is in this phase, he cannot move or attack physically. This spell likely costs an high amount of mana and used often should fatigue the caster.

When Eregos casted this spell, he also summoned Planar Anomalies - magical spark entities. We'll get to their nature later on.

 Spell Reflect aura - An other defensive ability - The dragons that use it have claws that hue with blue magic. Spells that were casted against them are bond to be reflected. This magical ability lasted for a limited amount of time. This ability is used only by old wyrms, such as Azuregos or Arcanagos. To be able to reflect the spells by enchanting or transmuting one's scales, must be a very difficult process.

 Blink and Teleportation. Blue dragons can blink and teleport, much like other races. They can also use this spell in an offensive way, to disrupt the distrubution of the battlefield: both Azuregos and Cyanigosa cast mass teleportation on their targets, bringing them close to their claws and their storm abilities.

 Shield of the Blue - Used by drakes in the Sunwell encounter and, later on, by dragonspawn (which indicates the spell is not hard to accomplish), the ability consists in a magical barrier which wards off all damage taken - or just significantly reduces damage done. It is extremely effective, being able to repel Kil'Jaeden's Darkness of a Thousand Souls! On the other hand, this barrier cost an high amount of energy and may leave a dragon drained.

 Malygos' abilities on occasion would cause Power Overload, which was an anti-magic zone.

Exploit Magic Abilities 

When they fight their enemies, blue dragons often exploit spellcasters' mana resources (or abilities) and turn it against them! Here, we see blue dragons' exploiting their control over the arcane to turn mana against their wielders. There are many abilities the dragons use to exploit their antagonists' magic. A standard mana drain used by whelps, a "mana destruction" (or mana detonate) ability, which basically destroys the enemy's mana and damages him aswell in the process. Alternatively they also use an arcane malediction.

In all cases, they exploit their enemies' mana (or magic) reserves, making so that their abilities backfire against them (i.e. Unchained Magic). They can also empower the abilities of their allies either through arcane breath (see Arcane Breath section) or by empowering their allies with arcane might (spellweaving).

 Mana Destruction - Detonate mana debuff. Much like burning a target's magical resources, the blue dragons may detonate a target's mana. The mana is turned against the owner, who suffers arcane damage over time! Furthermore, the caster is also bound to feel exhausted as his/her mana is being depleted. This ability is used only by Cyanigosa, an ancient dragon.

At first glance, it may have similarities with the sin'dorei's attempt to vampirize on magic, yet such is not true. For example, the mana is burned - not drained - and it is therefore lost (the blue dragon is not energized by it).

 Manastorm - Used by Azuregos, it deals frost damages and, for every damage inflicted, also removes mana points! Basically, a storm of enchanted ice which does not only deal damage, but every spike of also drains a part of the target's mana (magical energy). Surely such a spell requires a powerful wyrm to cast it.

 Unchained Magic - used by Sindragosa. Tooltip says: "Inflicts an arcane malediction on the target, causing any spells cast to result in a backlash of Arcane power after 5 sec. Multiple spellcasts by the afflicted target will intensify the backlash. Instant cast." It causes Instability: " Using magic while afflicted with Unchained Magic will build up unstable energy, dealing 2,000 Arcane damage to the caster per spell cast, 5 seconds after spellcasting ends. Instant cast. Stacks 99 times." Sindragosa marks the target with a spell (an "arcane malediction") who becomes unstable. The target is still capable of releasing spells yet doing so triggers the malediction: every spell which is released unleashes a backlash of unstable energy. What does it do? From the sound of it, it must feel like: you feel unstable energies coursing through you. Everytime you cast a spell, energy is evoked in excess, in a way you cannot control. Such energies backfire against the caster, damaging him everytime he tries to attack.

How does it work?

Perhaps it is a spell that affects the way a caster connects him/herself to the magical realm around him/her. For example,as you evoke energies from the ley-lines, you cannot stop and you are forced to summon a bigger amount of power, much bigger than the one required to release your spell. Hence, the spells you release are unchained, as they release an excess of magic that backfires against you! Indeed an excess of magic that backfires against the target.

Alternatively, a more simple mechanism is that it acts like an arcane mark (a malediction) which releases arcane radiations everytime a spell is released, hitting the carrier of the mark. Much like the arcane breath, it could leave similar radiations.

Very potent - used so far by a powerful wyrm such as Sindragosa.

 Wild Magic. Used by Kalecgos. This spell imparts either a buff or a debuff on player, hence the name "wild". It is an interesting variation of a dragon exploiting an unstable form of magic, as the effect is random and may backfire even against the dragon. It could be seen as some type of aura that affects all magical abilities in the same way, yet the result is unpredictable: it may either make their cast take longer, or take less time (etc, etc), or become empowered or weakened (etc, etc). Can be used by a young dragon.

Mana Drain ability. Used by whelps in Mazthoril, hence very standard as a spell. Drains mana, much like the warlock spell.

 Spellweaving. Used by the Aspect Kalecgos the Spell-weaver in the Madness of Deathwing encounter; this spell empowers the allies of the dragon with arcane energy, which is later released through spells (no matter what spell is used) against the enemy. There is a chance to blast the enemy target, dealing additional arcane damages, as any spells are released (triggers an arcane explosion). This spell was used by an Aspect, hence it is likely top-school.

 Aura of the Blue Dragon. Blue dragons' attunement to magic allows them to have a friendly aura, one that may replenish mana of their allies over time or increase the effectiveness of magical abilities. Such a spell was casted by Tarecgosa and also by Kalecgos during the Madness of Deathwing.

Arcane Abilities.

Like all wielders of the arcane, blue dragons can do pretty much what mages can do. Yet, we have seen before, they also have peculiar and somewhat iconic abilities such as the tremendous arcane surge.

 Arcane Surge. The dragons take a deep breath (or evoke much energy) before channeling a powerful surge of arcane might. It looks like a costant thunder of arcane which is focused against the enemy - a beam concentrated in the same direction. This spell is lethal and grows with power as the dragon does: for example, the Scalesworn Elite spell inflicted about 25k per sec every second, as it was channeled. Malygos' attack inflicted also AoE damage in a short area. Even the Scalesworn spell could kill a geared player in a single channel with about 60k arcane damage inflicted.

 Encapsulate: an Arcane Prison. Casted by Madrigosa against Brutallus, she summoned a beam which, as it touched Brutallus, turned into an magical sphere (prison) which, on occasion, would release an arcane explosion, injuring the one trapped inside. Brutallus had no way to escape it. Later on, Felmyst uses the spell on players. Used by an aged agent of Malygos, so likely requires some ability.

 Static Field - Malygos the Spell-weaver released this spell in the same location, summoning a cloud of unstable arcane energies which tear apart the air and everyone standing inside it. Likely, this spell is akin to an arcane lightning or fire in its nature (yet doesn't fill the body as electricity does) and rips body tissues apart, melting them in its power. Used by Malygos, hence a pretty powerful ability.

 Mystic Buffet and Arcane Buffet - Used by Sindragosa and Kalec, these abilities consist in an arcane explosion that deal an increased amount of damage as they hit the same target (stack, increase damage taken by 10% per hit). They have a wide range and can be casted often - even by a young but talented dragon such as Kalec.

 Amplify Magic. Used by Scryers, increase the effectiveness of arcane abilities by 100%. The spell is self-explanatory: all abilities against the target, who is marked by a spell, are amplified (i.e. both in damage and healing).

 Planar Anomalies. When Eregos shifts to the astral plane, he summons planar anomalies. These beings are much akin to the arcane sparks - likely a type of arcane elemental creature.They float around the area, periodically emitting bursts of dangerous energy. Planar Anomalies are said to "distort time around them", evoking what looks like an arcane explosion that deals an impressive amount of damage to nearby drakes. Rather powerful, used only by Eregos so-far. 

Comming "soon":

Crystal Magic. Crystal magic. More to come.

Conclusions from NPCs and bosses battles. 

Conclusions.

Blue dragons are not violent creatures and will not attack at first sight. They (usually) need a provocation to engage a challanger.

 Once in combat, blue dragons will fight from the air - if allowed - and release breath attacks and magical spells against the enemy. Their breath spells include frost breath and arcane breath. They are imposing physically and their big claws will often attack more than a single target (cleave attacks).

 When they release their arcane spells against the enemy, some choose to aim for a quick kill and do so in a direct form; releasing infamous beams of concentrated arcane energy (arcane surge) and arcane barrages and arcane bolts or frostbolt. Madrigosa, the Scalesworn Elites and perhaps even Malygos fought in a similar way. They fight primarly with arcane magic.

 Other dragons fight by controlling the battle-field, such as Sindragosa, Cyanigosa, Azuregos and Eregos. They will teleport their enemies in locations where they can suffer more damages from their arcane or frost spells, often marking the enemy spellcasters with spells that disrupt their mana or make their spells backfire against them. They fight with both frost and arcane.

Blue dragons seek to control the battle-field. They may resort to defensive spells: use spell-reflect, phase shift thus avoiding any attack and then summon entities such as planar anomalies which emits dangerous explosions of arcane energy or can amplify magic of arcane spells, increasing the outcome of any attack on the target. Alternatively, to control the battle-field, dragons also enchant the objects surrounding them (Kalec vs Kil'jaeden) or, much like Malygos when he destroyed the platform with an huge beam of arcane, change the actual enviorment so that it favors them rather than their allies. To gain control of the battle-field, dragons may also summon imposing storms of arcane or frost energy (mana storm, blizzard spells, static fields, Malygos evoked storms of lightning icicles) capable of dealing impressive amount of damages. Also, they usually do not conjure elementals when fighting and prefer their spells to have a more direct and immediate effect. While fighting, they seem to prefer transmutation, evocation and eventually enchanting abilities to illusion or abjuration spells. 

 Blue dragons are very susceptible to the surrounding magic and may be empowered by a favorable source of magic. During the Nexus War, they used the Surge Needles to take the monopoly of the ley-lines, thus empowering their ranks and launching a severe counter-offensive at the Wyrmrest Temple. Despite being outnumbered and eventually defeated, the blue dragons empowered became more powerful than their red cousins.

 Blue dragons also know of powerful defensive spells: the aforementioned spell reflect and phase shift are an example, but they can also summon effective but costly wards of arcane energies or mists of ice to protect themselves. Grown dragons are resistant to most forms of magic - especially to arcane and frost magic.

 In the role of support, blue dragons are not healers. However they may enchant the ground around them, so that it favors their allies. They can empower their allies with arcane energies and use auras that favor spellcasting. They can also amplify magic results.

 Their reliance on magic can also be a weakness, now that after the death of Deathwing they have lost their generic attunement to arcane. For example, in Azsuna the nightborne began to draw energies from the nearby ley-lines and prevent the blue dragons from accessing the lines themselves. This way, their spells became weaker.

 Out of combat, Blue Dragons are inherently capable of using arcane magic, as revealed by Stellagosa. They can sense the location of the ley-lines.  This is mentioned in both Legion and the Sunwell trilogy, hence is indeed canon.

 Apparently, they have the capacity to perceive strong magical powers as they are unleashed on Azeroth. This sensitivity to arcane magic, however, might be a talent only a few dragons possess. For example, Kalec did not sense Anveena's latent powers. Some blue dragons are good diviners, like the drake-champion Scryer, who could even sense stealthed units. However, as he was likely named Scryer after this talent of his, it suggests his divination powers are uncommon.

Many blue dragons are good enchanters. Like Kharmeera, Senegos, Kalecgos and other unnamed drakes involved in the Dragonwrath questline, who usually craft or enchant togheter many pieces to create a weapon or a ring enchantment. They are also good at conjuring items (chairs, beer even, tables) and can wield most forms of arcane magic, attaining the same powers of humanoid mages - if not even above them due to their inherent connection to the arcane, which is still with them after the Cataclysm.