Revealing mistake: During the end credits of this episode, a bowl of soup is shown. A frog appears in it for a few seconds and then sinks under the water again. You can see the string being used to pull the frog up. It surfaces as the frog does, and is very easy to spot.
The Punch and Judy Affair - S7-E8
Revealing mistake: When Mr. Humphries complains that the sauce is stuck in the ketchup bottle, one can see the silver plastic seal keeping it in, not some mysterious clog.
The Punch and Judy Affair - S7-E8
Revealing mistake: When the staff are assembled in Mr. Rumbold's office sans Mr. Lucas, Lucas comes rushing in late. However, before he does you can see him standing just outside the door, waiting for his cue to rush in.
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.