Limit break - only Blue Magic is stupid, although it worked in FFVIII due to lack of MP, and the fact that there were a few game breaking spells that really had to be limited (cough cough degenerator), there was no reason to limit it that way in FFX aside from how cheap the Petrifying breath could be

I'm with whoever proposed it earlier, a mix of both. Have your Blue mage and his list of regular magic that he can use at will (LvX Spells, Aqua/Fire/Whatever Breaths, Magic Hammer, Goblin Punch, etc) , then have his Limit Break unlock a set of "Super Blue Magic" which would include the cheap stuff like Degenerator, Mighty Guard, Shadow Flare, Stone Breath, etc