Steve, regarding British ballads collected from black singers:

Sharp and Karpeles noted down 'Barbara Allen' from Aunt Maria Tomes / Tombs, also 'Lord Thomas & Fair Ellender' and 'Sweet William & Lady Margaret' from Sinda Walker, who according to Sharp was probably of mixed race and 'sang very beautifully'.

'Our Goodman' / 'Three Nights Drunk' was in the repertoire of several black singers including Coley Jones, and was the basis for Blind Lemon Jefferson's 'Cat Man Blues':

'When I come home last night I heard a noise, asked my wife, "What was that?"
She said, "Don't be so suspicious, that wasn't a thing but a cat"
I've been all through the world, I've taken all kinds of chance
I haven't seen a cat come home in a pair of pants'

Not only did Leadbelly have 'Gallis Pole' in his repertoire; versions of 'The Maid Freed from the Gallows' were noted in several African-American communities according to Alphonso Smith, who reported one instance of the song's action being acted out.

I've an idea 'Sir Hugh' was sung by African-Americans as well.