I don't have any problem with a black actor playing Billy Bigelow, but what's been done so far is stunt casting. If you're going to cast Billy as black and leave Julie as white, their daughter damn well better be mixed race. Otherwise you're asking me to believe that biology doesn't exist. And you better account somehow for the radical change in the townfolks' reaction to what would have been heresy at the time and place where Carousel takes place. You could make all three black, which would ameliorate that townspeople reaction problem. A black couple would be far more likely accepted than a mixed marriage at that time and place. In short, I don't have problem with non-traditional casting if you do it believably. Just making Billy black without anything else fails the test.