Buster Keaton appears in blackface in several movies, including one that I have seen recently College. I have to admit that it was a bit jarring as I wasn’t expecting it.
However, when Downey appear in blackface for Tropic Thunder it’s pretty funny. So this poses an interesting dilemma – why do I act with shock at one depiction of blackface and amusement at another depiction of blackface?
While the scenes of Buster Keaton in blackface were funny, it wasn't caused by the appearance of blackface. Keaton is the Butt of the joke - so desperate to find a job to support his college career that he is willing stoop so low as to pretend being 'colored' to get a waiter job.
However, this comes into conflict with modern sensibilities - there is no shame in ethnicity, so the tension between what the Butt is implying ('colored' people are inferior) and our notions of what is acceptable ('colored' people could be waiters, customers or restaurateurs) prohibit the gag from succeeding in the way that was intended.
In Tropic Thunder Downey's character is also the Butt of the joke, but it is not because he is having to degrade himself. In fact, the intent is the opposite - the character is so conceited that he surgically alters his appearance. Downey feels that this simple act of reconstruction actually gives him the full past experience to actually make him black and understand the difficulties that a minority may actually have to endure.
We laugh at Downey because he believes that appearing in 'blackface' makes him more than he is. We fail to laugh at Keaton because he believes that appearing in blackface makes him less than he is.