Continuity mistake: Mr. Lucas says the figures for the previous Monday were 325 pounds, and at that point in the day the departments combined (170 and 150) were up to 320, five pounds short. However, later when Mr. Rumbold is talking to Mr. Grace about the sudden drop in sales, he says they are "still 20 pounds short" of the previous Monday.
Continuity mistake: Mrs Slocombe arrives with her hair in disarray, but once she walks away she only has time to put her coat away before she returns, by that time her hair is neat with no evidence of it being messy. She had to have returned quickly to hear Mr Humphries' story as she comments on it shortly after.
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.