Thoughts on blackface: Should it really be seen (by us now, looking at 1930s and 1940s movies) as demonstrative of the racist nature of those performing? In a few cases, but not all. The main place where I see it as being racist is in softshoeing -- and that in the same way as what Eminem talks about. A white guy just would not be accepted as a softshoer (in the same way that that white guy has had a hard time being accepted -- by whites and by blacks -- as a rapper). So the softshoer puts on blackface and at least pretends to be black, and gets accepted. The acceptance of white musicians/dancers in "black" performance types is growing, though; their acceptance among white people is in great part due to big-name white stars who joined jazz troupes or sang the blues (especially Bing Crosby, most notably when he takes the stage with Louis Armstrong's band in "High Society").
But as for blackface song&dance numbers, or individual black numbers (by black performers, not whites in blackface), both of which are often characterized as racist, offensive, degrading? Most of them (most notably "going to heaven on a mule" in "Wonder Bar" and the Lincoln's Birthday songs in "Holiday Inn") are lauding Abraham Lincoln and singing in praise of emancipation. "Going to Heaven" even portrays St. Peter and Archangel Gabriel as black. I'd say it's a pretty safe bet to assume they're not lauding Abraham Lincoln because of his little-reported racist remarks or disputed wish to ship blacks "back" to Africa. Are those songs lyrically expressing racist sentiments? A roommate argues that by their assumption that blacks would be the ones to be happy because of emancipation, they're racist. Now, that's just silly. It's like LGBT groups activisting about how AIDS isn't a homosexual disease, and then getting mad when other groups do AIDS stuff, saying that they're stealing a gay person's issue. You can't have it both ways. Many others argue that it's the white performers in blackface who make it racist (they didn't hire black artists) -- but one of the two Holiday Inn songs was sung by blacks, and that one's also called racist. Apparently, it's better just to have all-white casts. But that would be racist too. Can't have songs about blacks or about topics concerning them or in styles associated with them without having black people perform them, and having black people perform them is stereotyping, and not having the songs at all is overlooking them. Goodness!