Revealing mistake: At the beginning of the Scottish dance when Captain Peacock is demonstrating the sharpness of the swords, he first cuts through the center of a newspaper page held up by Mr. Spooner. However, you can see that, just before the sword makes contact, Spooner actually tears the paper apart down the middle.
Continuity mistake: In the ruined photo taken of Old Mr. Grace and his nurse and secretary, the secretary is looking away from the camera and her leg is raised. However, in the scene preceding, she was looking at the camera and raised her leg after the flash went off. Also in the photo, both of Mr. Humphries' feet are in the air, though one was on the floor when the picture was taken.
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.