The Father Christmas Affair - S4-E7
Visible crew/equipment: When Mr Humprhries is talking about the whereabouts of Mr Grainger, explaining about the inability to remove the Al Jolson make up, you can see John Inman looking up before having his face shadowed by a boom mike.
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.