I went to a show once on the Edinburgh Fringe, billed as a celebration of spirituals & the roots of the blues. It was very much as you'd expect - re-enacted scenes of plantation life interspersed with song - with the unexpected twist that the company (of three) was white, middle-class & well-spoken, and blacked-up.

We were just horrified - it's hard to convey what an offensive travesty it was. The Edinburgh Fringe being what it is, there were only about six people in the audience - and after about ten minutes we all walked out. My only slight regret (apart from the waste of ten minutes) was that the only black person in the audience had walked out first, on his own, & presumably never realised that the rest of us hadn't put up with it either.

Blacking-up aside, I feel pretty much the same about doing songs from any culture that's not your own. Your folk-singing voice should be your voice; if you listen to yourself singing and it doesn't sound like you, don't do that song. Equally, if you make it sound like you and it makes the song sounds ridiculous, don't do it. As an English singer doing Scots material, for example, you can usually turn 'alane' and 'gane' into 'alone' and 'gone' without doing too much violence to the song. If you can't, well, there are plenty more songs.