It’s an interesting question. There’s nothing really inherently offensive about it, but I gather that in the Jim Crow days, comedians would don blackface in order to make fun of black people. Assuming that’s true (as opposed to, maybe it was good-natured humor), how far into the rear view mirror does that need to get before it’s OK again?
Blackface had its origins in minstrel shows, on southern show boats, etc when blacks could not be integrated into white casts. This is illustrated wonderfully in the original sound film of Show Boat in which Irene Dunne plays the banjo in blackface. She’s very funny. Even Paul Robson laughs at her antics. “Can that girl shuffle!”