Isn't there a difference between blackface and painting your face black? If he paints his face like an exaggerated cartoon of a black person I think that's totally offside but if he's just using black make up to look likes black person I think it's no offensive.
That being said I would consider your audience. A house party with a bunch of friends might be fine but if you go to a pub or club someone may take offence and then you have a big crap storm on your hands.
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