When people say "Blue Mage"...

[KikoriL]

I keep wondering if it could be a Magic-based tank. Instead of just outright learning spells forever like traditional Blue Mages, perhaps taking a FFX-2 Gun Mage's approach to having ways to deal with different specific enemies then and there.
Consider a flexible, reactive defensive buff that increases resistances to recently received damage types & elements, and decreases the potency of many ailments. Add a way to adjust damage type dealt with standard attacks / skills to hunt for an enemy's weakness. Combine this with a way to absorb and rebound enemy skills (even if it's just soaking the skill's potency, damage type, and element to reflect), and it seems like it could be Blue Mage'y enough to work.
Maybe?

[Steady]

I think Blue Mage and Mediator should just come together.

Have a class that learns abilities from Beastman tribes.. like their enspells, haste, that sorta thing.

Blue Mage in FFXI was the closet thing to a Blue Mage Tank, and it was a Tank like Red Mage was. Not intended, but could manage.