It's really a question about dialect rather than race, isn't it? I'm assuming that the singer speaks (approximately) the same language as the racially different person.

The only difference between this and other dialect issues is that it's more likely to be seen as patronising or offensive if a white person sings a song in a "negro" accent. To an extent the same applies to, say, a Londoner attempting to sing in a Geordie accent, but that is more likely to cause mirth than to cause offence.

Now I think about it, it happens all the time in popular music, though, doesn't it? Many of the traits of musical style, accent and dialect used in R&B, blues, and jazz are derived from black music and speech, but that doesn't stop white singers from trying to sound black. Is traditional song different, I wonder? And if so, why?

Marje