When Robert Downey Jr. did it, the context mattered a lot. It was a commentary race. There was an actual black actor along side him that was basically spelling it all out for the audience.

They were using blackface as a way to talk about race issues rather than the offensive way it has been used in other contexts.

Whether or not that kind of thing would fly today is a different question. I certainly hope that people could look at things holistically and not just have a knee-jerk reaction to something like this. I'm also interested in what black people think about this, because ultimately that's what matters most.