I don’t know if it’s a kind of blackface, but recently I listened to some old time radio episodes (Duffy’s Tavern) and the way one character is portrayed made me cringe a bit - it’s not that he says stupid things (though he does); he seems to be deliberately playing a mentally-handicapped character (Clifton Finnegan) for laughs, like a middle-school student might have done to mock other students when I was a kid. Distasteful.