It's one of those double standard things- like most things to do with race.
BUT it does date back to when black people were not allowed to work in entertainment, so a white person would 'blacken' their face and pretend to be black- acting like an uneducated hick too.
As I said though, it is a bit of a double standard. There was a time when women were not allowed to perform on stage and men would dress up as them if a female character was needed. When that is done now, noone freaks out.