I'm kind of on the fence about this as well. I think it's important to note that he's not playing a black guy per se, he's playing a white guy playing a black guy...which seems like a kind of silly distinction, but in this case I think it's significant. It seems that they are lampooning and satarizing blackface--and Hollywood in general--more than they are taking part in blackface. I don't think we're talking C. Thomas Howell in Soul Man here.
That said, however; these instances of blackface as satire or parody seem to be becoming more prevalent and I think they serve, somewhat, to desensitize people to the white-person-in-black-face-paint phenomenon, which I don't really like to see. I think even when something is done with good intentions, you can't ignore or forget the history that it comes out of. I don't think it's necessarily a good thing for us to take the taboo off of blackface even when done in decent taste because it's a short step from decent taste to really, really bad taste. My fear is that something like this could lead to a bunch of "Hey, Robert Downey Jr. did it in a movie, why can't I do it for Halloween?" idiots.
But at least this is generating discussion and making us think, which is antithetical to the history of blackface, which was to not think. Generally speaking, blackface audiences didn't think critically about or consider the impact or consequences of what they were watching, they simply confirmed awful stereotypes and reduced serious, painful, real struggles to knee-slappers.