I'd have been horrified too, Pip.
So this was not billed as a re-enactment of The Minstrel Show?
The songs of the Minstrels from their birth in the 1840s were not the songs of African Americans. They were songs composed by singer-songwriters like Stephen Foster and the early Minstrels themselves. Performers at the time also sang songs in the stage-characters of Cockneys, Irishmen, Scotsmen...
Along with the characters came the accents. Theatre has always dealt with stereotypes. Always there has been the potential for curruption of theatre into personal attacks. The potential for cruel caricature was greater in the case of the Minstrels, because of slavery -- but the Minstrel Show played a huge role in bringing the changes that ended it.
Joy