It sounds like most black mages aren't very powerful, as in the effects they can place on the enemy are very minor. MC-kun is the exception, having been trained by a Demon King, and can place exceptionally powerful effects on his opponents.
Really, what he should have done was challenged his party to reclear a floor without him, 4 manning it. If he was truly a burden, they would have an easier time. If he was neither burden nor boon, they wouldn't notice a difference, and if he was helpful, they would have a harder time of it. With that evidence in tow, the Warrior would have no choice but to back down. But MC-kun gotta have imposter syndrome or there'd be no story.
Really, what he should have done was challenged his party to reclear a floor without him, 4 manning it. If he was truly a burden, they would have an easier time. If he was neither burden nor boon, they wouldn't notice a difference, and if he was helpful, they would have a harder time of it. With that evidence in tow, the Warrior would have no choice but to back down. But MC-kun gotta have imposter syndrome or there'd be no story.