Time: late 70s and early 80s.
Although not an all encompassing statement, the “blackface” thing was pretty much played out north of 37. The humor was gone for several reasons, the result of “The Great Migration” being chief among them.
It remained in the south and slave states because of the remnants of copperhead democrats, Klan members and the old redneck cultures still around.
I think by that time if it was done it was largely in an ‘ironic’ manner, like the protected classes still do today (HRC “they all look alike”). I’ve been afraid they were going to start reaching back way way way back in the past, hell they’ve been doing it for some time. Thank god *I’VE* never had a racist thought.