Black Face is considered offensive because it's a ridiculous parody done solely to demean the African race. Whereas Robert Downey Jr. was legitimately playing the part of a realistic, badass character.
WHAT DO YOU THINK OF THOSE HUMORLESS EVERYTHING-IS-OFFENSIVE, EVERYTHING-IS-RACIST, I-LIVE-TO-BE-OUTRAGED POLITICALLY CORRECT TYPES? HEY, ROBERT DOWNEY JR. THEY WENT FULL RETARD