A black mage’s unique contribution to a raid is how much higher their dps is than the baseline. That comes out not just in hitting harder gcd for gcd, but also in getting a bigger absolute benefit from other party members’ damage buffs.

There definitely exists some margin by which “greedy” dps like blms could exceed “generous” dps like bards that would make them worth bringing. For instance, if a black mage did literally double the damage a bard or dragoon boasted, the vast amount of damage that ast cards, dragon eye tethers, etc could add would outweigh the benefits of bringing yet another party buffer. The question is, how big is that margin?