Yeah, the movie is about actors making a war movie in a jungle in Asia and accidentally getting mixed up in a real conflict.
RDJ plays a white, blond, blue-eyed, Australian method actor who was cast as a black character. As a method actor, he stays in character the entire duration of the shoot, so he’s always in blackface, acting as his character. Early in the movie there’s a montage of his other roles, so the audience and all the characters are fully aware that he’s a white Australian in blackface going overboard with his method acting.
In the movie he says he doesn’t drop character until after the DVD commentary. I’m the commentary for Tropic Thunder, RDJ is in character the whole time.