Cold Comfort - S2-E2
Revealing mistake: In the very beginning of the episode you can see Mr. Grainger sitting in a chair to the very far left, with his head down, waiting for his cue to walk-on.
Cold Comfort - S2-E2
Other mistake: At the very beginning you can see a girl in the background with long hair, pausing to speak and gesture to someone off stage, before she removes the sheet draped over the counter.
Answer: In minstrel shows there was also a stereotype of black men preying on white women, so that's probably why she was not in black face. Also, Mrs. Slocombe is in blackface because she is an older, stout woman and she fits the "mammy" stereotype.