From what I have come to understand, not being a major Final Fantasy buff myself, the Blue Mage is a class that learns abilities from their enemies and uses them against them.
With that said, if Blue Mage were a tank it feels like they'd be a heavily vengeance-based tank; a lot of abilities that reflect damage like WAR's "Vengeance," but also an ability that maybe works like Jutsu where the command changes based maybe on whether or not the Blue Mage has been hit by a magic or physical, elemental or holy/darkness attack.
It would also be a mage tank, which I also think would be cool.
But I am no game designer, I'm just spitballing ideas ^w^