Tank would be the most fitting role for BLU in the game as it is right now since tanks are the most versatile role.

They have damage skills for enmity, allowing BLU to learn attack spells like 1000 needles.
They have healing skills for recovery, allowing BLU to learn healing spells like white wind.
They have mitigation skills for defence allowing BLU to learn mitigation spells like mighty guard.

I really don't understand why people want BLU to only learn their spells in job quests, that would be incredibly boring. BLU could easily have a basic skill set using their weapon and learn blue magic on top of that from monsters in the world.