Revealing mistake: The cake the groom's father has is treated as if its real, but when smashed over Mr. Grainger's head it's obviously Styrofoam and hollow.
The Father Christmas Affair - S4-E7
Revealing mistake: At the end, when Mr. Grace is inspecting the Father Christmases, you can see his secretary behind the scenery waiting for her cue.
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.