I don't know the fiddler from Nova Scotia to whom Rick Fielding refers.
However, none of the other black stringbands and musicians that have been listed in this thread have anything to do with bluegrass. Old-time and early blues, yes. Bluegrass, no.
To be honest, I've been to about 50 or 60 bluegrass festivals all over the country in the past 20 or more years and I have never seen a black musician playing at any of them.
I don't think that I've even ever seen a black person in a bluegrass audience.