To be fair, it kind of makes sense why entertainment-wise a black mage would be more suited to fighting heroes than monsters. Monsters are relatively low-intelligence species that don't seem to know human language. If you make a dumb goblin dumber or blind or something, everyone else just sees a dumb goblin being a dumb goblin, and so it seems like a black mage is doing nothing. But versus intelligent foes, say heroes, if you make them blind they'll call out they cannot see, and if you poison them they'll cry in agony as poison burns their veins. From a business perspective, black mages should really be concentrated on the "evil" side. Whether this would make them popular or unpopular (like heels in wrestling) would depend how commodified this form of entertainment is amongst both monsters and humans.