Yeah I was going to write about that as one of many things that was wrong about this movie. I will say the blackface bit was probably the best musical performance of the show and it was right at the beginning.
Beyond that the movie never developed any momentum even tracing a trip to "stardom." There were lots of difficult to believe parts such as how a 30-piece orchestra is assembled and rehearsed in time to accompany a guy making a last minute appearance, where his stage clothes came from, why he's obviously too old to be playing this part etc. It was fun to drop in on 1980 again.