You can't. Any idea that has BLU retain its signiture idea would revive long dead (for good reason) problems with the game. The cross class system from ARR for example, required that players go out of their way to obtain skills from other jobs that were vital, leading to WARs and DRKs going into raids without provoke. Learning skills from monsters (BLUs core identity) would bring this problem back into the core battle system.

The solution would be to give BLU all its spells through levelling, which makes BLU nothing but a BLM with extra lore flavour. BLU doesn't have a gameplay identity, it is, and always has been a sidequest activity in whatever game it was in, and that has been accurately portrayed in this game.

The only issue in current BLU is accessibility and availability of content. BLU should be 80, BLU should be able to do dungeons, Potd, Eureka, everything. But it should never be allowed to do raids and ultimates