The biggest issue with making BLU into a regular job in XIV is that it'd more or less just be a slightly different flavor of Black Mage.

And they'd have to get rid of a good amount of spells... Stuff like Final Sting, White Wind, Bad Breath, anything that causes statuses like Frozen, Paralysis or Petrify, Level 5 Death... And others depending on what role they would want it to be...

And also have what few spells remain be learned by level up instead of the standard BLU method of learning from enemies.