Of course if one portrays a criminal, or an old fashioned style "coon" impersonations... yes very Racist.

But if someone wants to dress up as nick fury from the avengers (Samuel L Jackson), which is no way a derogatory or comical role, in fact a very serious and cool character, what is wrong with it?

What if someone dresses up as MLK in a non sarcastic way, he was a prominent American hero after all, why would they not wear some form of black face? And how would that be considered racist?