In comedy, the boundaries are different. Am I the only person that saw Jimmy Kimmel dress up as Karl Malone on "The Man Show"? His costume was complete with body makeup.
"Black Face" is different than body makeup though (although there are some that confuse the two).
Above is an example of what blackface looks like.
As you'll notice, the jackasses pictured above have put what I guess is ash on their face (which really doesn't resemble any real skin color). They have accentuated certain aspects such as the lips. That is blackface. Body makeup is NOT blackface.
Blackface has a certain heinous history. Face makeup, when worn to make fun of a race and stereotypes is wrong. When worn to make fun of an individual (like Karl Malone), it isn't the same kind of thing.
In the case of the Wayans, since I haven't seen the movie yet, I'm not too sure they were making fun of the white race. And to tell you the truth, I wouldn't care if they did. No skin off my back. I usually find that kind of humor to be funny. I'm sure they're making plenty of $$$ off of this somehow contraversial movie. We talk and the make bank.