Johnson uses sarcasm in this poem as he expresses the opposite of what he means. When he says "pile on the Black Man's Burden", and gives examples of how people can make black men more miserable than they already are ("his wail with laughter drown"), he is using sarcasm. He clearly does not want people to pile on this burden and make black men's lives harder, but he is saying that people should do it to show them how ridiculous it sounds and to point out that people are already doing that.
