I have been debating whether the blue mage should be a tank or dps. To me tank seems like the most logical choice. Most tanks have their own little nitch. The blue mage will use blue magic to damage and inflict debuffs on enemy, with "Bad Breath" being one of the most powerful skills. Perhaps learning a healing skill like white wind.
I can see the blue mage going out during their job quest and slaying beast to gain new abilities. Slaying coeurl to learn how to paralyze. Facing a Fenrir to learn frost breath to freeze weak enemies. Going into instances to kill weaker versions of Behemoth class fate bosses. Many people think that a Blue Mage should get a sword of some kind. I, on the other hand, think that we need a little variety. At first I thought of a whip, a call back to Quistis from ff8. But now I think a crescent blade would work best. I got this idea after seeing the Azure Witch in Mobious FF. I can even see the Azure Witch being a main part of the job quest.