Similar to the Warlock Green Fire quest chain, why not give mages a quest chain to unlock blue fire for their fire spells.
Have the mage player go on a quest to locate a blue dragon who's unlocked the secrets of blue fire magic. You must defeat her in fire spec so that you may prove yourself worthy of her knowledge.
Here's the kicker...
Have the dragon be called Azulagosa and be voiced by Grey DeLisle, who's already voiced characters across both Diablo 3 and WoW.
You may not get these references if you haven't watched Avatar the Last Airbender, shame on you, but I'm sure mage players would enjoy blue fire nonetheless.