Well here's an example of some issues I see with BLU:
1) Accuracy and Damage of magical blue magic is disproportional to the amount of MP it costs: other than breath spells, the rest of the magical spells are totally worthless and do nothing but serve as a vessel for auto-refresh. Physical magic costs up to five times less the MP and does more than 10x the damage in most cases against exp mobs and HNMs.
2) Unless a BLU is subbing THF, most physical blue magic damage sucks versus endgame mobs.
3) Diffusion should be a 5 minute timer, with the remaining merits extending the duration of the spell cast.
4) Assimilation should give 1, 2, 2 points for the merits, to make it an even 5 extra points when maxed.
5) TP Bonus merit is fine. Convergence should do a little more than what it does (seriously, with one merit I notice that burst affinity does more for my spells than Convergence does).
6) Azure Lore would be better served in an emergency situation if the entire Blue Magic spellbook was unlocked, as well as recast timers being reset when used - also... physical spells should automatically gain TP bonuses and magical spells should gain burst affinity bonuses... minus the ability to skillchain and magic burst.
7) It would be nice to have an MP Recharge sort of spell similar to self-refresh... where it costs 75MP but restores 150 at 10/mp every 5 seconds. That way BLU can refresh every once in a while with diffusion and gives them the ability to restore MP if needed in a backup role (something currently only RDM, COR, and BRD can do). The spell could even cost 5 BMP and it would still be fairly balanced.
8) Also, having a better, higher level AoE Cure spell (along the lines of White Wind, where it's an AOE healing breath attack that heals based on current HP for high enmity) would also expand this role.
9) Give BLU throwing skill, since they have so many throwing chakrams and no skill to utilize it.
10) All new jobs, BLU/COR/PUP/SCH/DNC need some sort of JSE set that doesn't suck... something that is on par with the Barone/Hachiman/etc sort of JSE sets so that these jobs have better end-game gear. As it is, it's pretty much the same as it was before other jobs - partial AF, partial abjuration.