Blue Mages must be within a certain level range of the monster, and be able to get some amount of experience from it in order to gain its ability.
Did anyone bother to ask if learning abilities from NMs would be possible?
The other thing is, are there any mob types that aren't high enough level to give exp to a LV75? (The possibility for a BLU to get screwed from learning an ability forever would suck.) Now that I think of it, how would this work in capped CoP zones for mobs such as hippogryphs, etc., unless they just won't use any BLU abilities from those (AoE Dispel WS from Hippogryph would be pretty pimp though).