I disagree on BLU's architecture not being good for an MMO. BLU design doesn't have to be all or nothing in terms of what players can learn from mobs, though the developers seem to disagree with me as they think BLU has to learn ridiculously OP spells in order to live up to the concept.
I mean barebones without mechanics, you could have BLU with 27-28 ability slots like a standard job. Have a standard set of monster spells they have to unlock by killing corresponding mobs. Use the spellbook architecture currently in place to have certain spells be swappable. For example, the lv35 job quest spell could be White Wind, but the spellbook will show you can swap that for other spells that count as utility skills (maybe a nerfed version of Mighty Guard, or Moon Flute), provided you've done the legwork to unlock them. For content like the Masked Carnivale, let the BLU get access to even more spells that they would not be able to use in duties/dungeons/raids.
And in case anyone would say "it'd be unfair for BLU to be a regular job and have content dedicated to it", I'll point to the fact that I'm the guy that suggested BRD get its own content because it makes sense in terms of lore and job concept, so... ¯\_(ツ)_/¯