While individual musicians of African ancestry undoubtedly made significant unsung contributions, I would say that whatever impact they made, they made as individuals -- not as representatives of a distinct ethnic subculture with a life of its own.

This is a generalisation but it comes pretty close to describing what is looking to be the main difference between the US and UK. I think it would also be generally true that there has been little attempt to exclude anyone from contributing to folk music (or any other music) in the UK. While black people in the US may have greatly influenced all US music (then the rest of the world) - in the UK it is probably more the case that black people have contributed directly to it - in pretty much the same way as every other individual here has done.

Or this was the case until fairly recently. There are signs that in areas where there large groups of younger black or asian people - these are starting to develop separate musical genres.