I have to say that I am enjoying this manga quite a lot. I do like that the Hero Party are not all asshats or the tired old cliche of the Hero, his harem and the MC. They even had legitimate reasons to question the Black Mage's addition to the party since his spells have no visible indicators and that they feel nepotism is the main reason Phoenix added him to the party.
Since adventuring in this work is pretty much a reality TV show, I can see why less flashy classes get pushed to the wayside. Smarter parties might actually have their Black Mage doing something; running commentary, calling out tactics, evil gloating, looking pretty etc. This justifies their addition to the audience while the party appreciates their debuffs.
I can easily see why the Hero Party wound up in this state. They had taken Remme's debuffs for granted for so long that they started believing that it was entirely their own strength carrying them through the dungeon. They might have noticed the difference at the beginning but after a length of time, they figured that they had outgrown Remme's debuffs and were that kickass all on their lonesome. Essentially they did the worst thing a professional competitor could do, they bought into their own hype.
I also like that the Hero is actually a nice guy and not the kind of person that makes the audience ask if they had gotten the Hero and Villain titles mixed up somewhere. I am kinda disappointed that he did not also quit the party since they were adamant about kicking out his Bro. What did you think would happen when you denigrate and expel a person's best friend?
One thing I did notice is that it was the Meatheads, those who rely entirely on physical ability, who failed to realize the power of Black Magic. Not only is it a potent force multiplier, it is incredibly subtle. The enemy might not even know that one of their own is afflicted and in serious trouble until he is cut down in one swing. The stalwart warrior who you could expect to dodge arrows gets rooted to the spot all of a sudden.
This plays well into Remme's new role as a Demon Army employee. It is not like he is an actual traitor to Humanity as it would be in other works. If one side kicked you out, how could they blame you for switching teams? Remme could just sit ominously on a throne somewhere, swilling wine in a goblet as the monsters clean up the Adventurers. Few would even pay him any attention since most are unfamiliar with the effects of Black Magic (since Black Mages are apparently rare and underappreciated), allowing Remme to act with impunity. His first assignment? Why the fifth floor of course. He will probably wear a mask to disguise his identity to give him that mysterious air. Soon that level becomes the gate which adventurers cannot overcome. All parties' progress stalls at the fifth level.
When he is not stymieing Adventurers on the fifth floor, Remme is likely have innocent dates with Mira who is probably his forgotten childhood friend who has had a crush on him since they were wee kids.