I don't really think this is a fair analogy. In so far as we can say Bert Williams was of the minstrel tradition, he certainly wasn't a fan nor its originator. I'm sure if he had been given the choice, he would have done something far more dignifying, but there wasn't much else for a black man to do in the theatre at that point except to dress up in black face as the white folks had been doing for decades. I'm sure you know more about the innovations he introduced into that kind of vaudeville, but his is hardly a comparable situation to that of Cohen and Borat. And seeing or knowing about Williams does little to make minstrelsy as a style of humor any more acceptable or less problematic in terms of the racist and nativist overtones in which it inevitably trafficked.
Along these lines, I'll also mention that there is a controversy brewing here at GMU because a white student went to a Halloween party dressed in blackface, wearing chains and torn clothes. I'm sure this could be spun as some sort of subversive act, but some things can be considered in such bad taste that they are simply not funny. Story w/ video link here:
http://www.nbc4.com/news/10294493/detail.html
-s