Mayhew's informants, the 19th century street singers of London, seem to confirm what McGrath says. They talked about learning from each other how to perform the latest fashion in singing, dancing, characters. At the time, they said, this meant American Blackface Minstrel characters. There is no suggestion of mockery of real Black people as far as one can tell from reading their words. The characters who were mocked were British upper-class twits and pompous asses. The characters, many of them well established, that were played for laughs were "Clown" "Jack Pudding" "Silly Billy" etc.
Cheers, Joy