As far as a nerf in optimal performance for a BLM... I'd disagree.
If there's a weak monster and BLM can go to town on it and blast it away into nothing, that's fine.
But resistance is a problem. BLM's MP limitations and low life/defense is not sufficiently balancing the potential damage output, because quite simply their MP regeneration is high enough to keep their damage consistent with a Bard (who will be there anyways given their universal support) and the fact that most enemy attacks are better maneuvered around by ranged classes.
So basically they are getting all of the damage, for little of the dangers to keen Black Mages scream about as their primary balancer.
My proposal on this is pretty simple: Make things dangerous for the Mages again.
I liked the concept introduced by Garuda's Plumes - Low HP, highly magical defensive minions dispatched by boss monsters. If there were more fights that introduced monsters like these that targeted ranged classes, but were suitable to a quick death from the Melee Classes, this could balance out importance or roles.
I also mentioned before more things that would attack the typical staple of attacking at range, like AoE's that are safer when you are close to the monster - or scripted switches in boss battles that ranged claseses would need to stop DPSing to use., damage to MP, etc.
There are more ways to balance the game than simply nerf the optimum performance of a class, though I do believe the current game now lends itself to ranged classes a bit too much. But I also firmly believe that empasising the importance of a varied party though fight mechanics is and class buffs is a better overall solution.
For the current fights, however? Perhaps some spiked Magic resistance when magic is overstocked against high difficult targets. That would help reduce some of the Black Mage stacking, at least.