If BLU was going to be made a full job, it would not behave like it does now. You will learn your spells through job quests and via level up (the job quests acting like, oh, you see this enemy use this ability, you learn it). It would be stuck as a caster DPS and so you would lose alot of utility, such as pom cure, gobskin, white wind, diamond back etc. It would have a rotation you can build to optimise damage, which will be dictated by the spells you learn from level up/job quests etc.

The problem is, you have two choices. You either make it a full job, in which case, you lose all the versatility the job has and make it just like any other caster with a rotation, or, you give it the freedom to do whatever it wants, it can have the powerful abilities, you can have the really strong heals etc. however, you have to sacrifice its ability to do current endgame raids so that you cannot use BLU to potentially cheese mechanics and make the fights trivial.

You can have one or the other, but not both at the same time.