I believe the mandolin player with Martin, Bogan and Armstrong was Howard Armstrong, who died a few years ago at the age of 90-something. He was quite a character and an amazing blues mandolin and fiddle player.

And yes, there is a rich black stringband tradition that is pretty much dying out now-- I think Howard Armstrong and Yank Rachell would have been among the last that we know about who were still playing blues mandolin (and in Howard's case blues fiddle).

I think there are some documentaries and albums of Howard Armstrong around -- try googling.

Eve