The whole deal with black face is not just something like a person pretending to be another race, it is specifically about the history of black face. It was where for a good amount of history black actors would not even be hired for black roles, followed by elements of mocking caricatures of black people as a minority group, a low blow of punching down with quite the history.
I personally care less about Robert Downy Jr's portrayal in Tropic Thunder, since a good deal of his character is mocking the type of actor that would the ridiculous thing he is for a role. It is ridiculous in the universe of that movie that someone picked him for the role over a black person. But I can't tell someone else, especially if they are black, whether they should feel offended by it themselves, it is not my place as also a white person.
As a non-binary person, I probably could speak out against Benedict Cumberbatch's character in Zoolander 2 though.
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Through dream I travel, at lantern's call
To consume the flames of a kingdom's fall