Pennington is a moron. Only the uneducated consider bluegrass to be nothing more than unsophisticated hillbilly music. The difficulty, complexity, and history of bluegrass (it is a blended musical form with many ethnicities contributing to this uniquely American music) is appreciated around the world and around the US, particularly among musicians and music historians. It contributed enormously to country as well as rock n' roll and other genres.
Bluegrass has also enjoyed a revival among college students, lay people, trendsetters, and those who live in cities out West, NYC, Chicago etc. as well as those who are very much into the music scene.
As to black fans who dig the song... the banjo, in particular, is a black American contribution to American music. Black folks very much influenced and contributed to bluegrass, country, rock and other genres in MANY ways. Contrary to popular belief, no music develops in a cultural vacuum and all Americans can claim American music genres as their own as just about everyone contributed to the development of various genres.