Nah. It's not the "black" I'm opposed to, it's just changing the character's race at all. Black, Asian, Latino, wouldn't matter. Superman is a character that everybody knows. You go to the most remote, poorest village in the free world and say the word "Superman," and everyone is going to instantly know who you're talking about, and a very specific image is going to come to their mind. Red cape. Blue suit. Big S. Spit curl. White dude.
I've never heard anyone complain about Superman being white. I've never heard anyone accuse his creators of appropriation (in fact most folks know Superman, created by two young Jewish artists, is a metaphor for their experiences growing up as Jews in America). Superman is what he is. There is no sense in changing it. The only explanation I can think of is WB trying to pander to black audiences.
But if they really want the black audience, why not simply adapt a black character? Marvel did it with Black Panther. And BP wasn't a character general audiences were even aware of, until he showed up in Civil War. DC has plenty of their own black characters they could adapt. How about a John Stewart Green Lantern flick? A Mr. Terrific movie? A Martian Manhunter movie (granted MM isn't actually black but chooses to appear as a black man, which itself is a powerful statement). Maybe a GOOD Steel movie.
What the WB fails to recognize is that the black audience -- and the white audience, and the latino audience, and the Asian audience, etc. -- love Superman as he is. Change too much about him and he's no longer Superman. That's why there was so much of a backlash against Man of Steel. We all know Superman doesn't kill, and he certainly wouldn't have stood by and allowed his dad to die in a hurricane if he could have stopped it. If fans can look at that movie and say "Yeah, that's not the REAL Superman," what do you think they'd say if he was a different color?