Actually, Blue Mage would make a pretty good Magic Tank, but different reasons. I could see it as the sort of tank that eats AOEs or mobs. Bad Breath for a bunch of debuffs to draw aggro, White Wind to heal everyone a bit(though have it focus on drawing aggro more), and Mighty Guard can serve as a handy def/mdef buff or maybe a sort of damage resistance that negates a certain ammount of damage per hit to deal with mobs of weak enemies/attacks?

Or maybe a move that draws in AOE damage to the Blue Mage and make it a harder hitting attack to the tank? Of course, add attacks like goblin punch and detonate as standard attacks/combos as well.