Q, The book "London Labour and the London Poor" by Henry Mayhew (the section on street entertainers) gives a few clues about mid 19th century English performers' attitudes to Blackface minstrelsy. Mayhew interviewed his informants at the very beginning of the fashion. A lot has been said since then about Blackface performance but Mayhew was writing at the time!
"The Cowells in America" edited by Willson Disher is also a good source of information about the attitudes, in America, to Blackface performance. It's much more than this of course. It was also written at the time. The author, Mrs. Sam Cowell (her own title for herself) was a friend of many entertainers in America and England. She was touring in America when the Civil War began. Her stories about the bravery of entertainers who spoke out against slavery, from stages in the South, makes interesting reading. Her cousin barely escaped with her life, and lived in exile from her home in North Carolina, for the duration of the War. She, like many others, continued performing in the North.
Interesting reading both books. Cheers Joy