I have seen many people put on blackface and call it a joke. Most of the time the joke is, "Hey isn't it outlandish I am doing this? Isn't it funny that since we all know that I am not racist that I actually am doing the most racist thing possible?" Sadly, I have never seen it be funny.
I'm not saying that it can't be funny though. I am a firm believer that you can make anything funny and that absolutely nothing is sacred. I believe the key is being smart and responsible, and then on top of that... funny. More specifically, if you are going to go out on a limb about race or whatever touchy subject that you are dealing with, then you had better really address it well in your material.
I think that the sketch on thirty rock last night with John Hamm and Tracy Morgan covered all of those qualifications and so for the first time that I can remember in my life, I found a sketch with someone in black face hilarious. Way to go Thirty Rock.
Check it out.