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Painting your entire body to be pitch black is not like blackface, blackface usually involved someone putting on makeup in a very specific manner in order to over-exaggerate certain facial features in order to look like a caricature of a black person (one standout area being the mouth, in which usually the mouth region would be a bright red in order to resemble "huge lips," and the eyes would be a real standout feature as well), since the whole point of those blackface minstrel shows was to often play up various stereotypes of black people at the time from a white perspective (black blackface entertainers existed, but I'm unsure if audiences back then could tell if a blackface performer was black or white).

Also let me add a Jontron clip to this comment: