A Change Is as Good as a Rest - S5-E2
Visible crew/equipment: In the opening scene in the board room when Mr. Lucas is commenting on how Mrs. Slocombe is "big in the south," you can see the shadow of a boom mike floating back and forth across the table.
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.