If he was just sporting a random blackface then yes, I could imagine that would be offensive .. but, he was with a group of people all dressed as characters from a film. Was he not just playing his part? same as if me and my friends were going as Simply Red, I may wear a ginger wig (flippant I knew but my ginger DS gets more abuse for is ginger hair than his dual heritage BFF)
But, I'm not black so whether I'm offended or not is irrelevant.
Maybe because being 'racist' to me is thinking of, or treating someone differently because of their colour, and I genuinely don't, that's obviously too simplistic a view though.