Sonny Boy Williamson
There were two Sonny Boy Williamsons. It's unsure to which one Zappa referred (though presumably the second one).
Sonny Boy Williamson (John Lee Curtis Williamson, March 30, 1914 — June 1, 1948) was an American blues harmonica player, and the first to use the name Sonny Boy Williamson. His best known song is "Good Morning Little Schoolgirl" (1937).
Sonny Boy Williamson II (Aleck "Rice" Miller, March 11, 1908 - May 25, 1965) was an African American blues harmonica player, singer and songwriter. His best known songs are "Fattenin' Frogs For Snakes" and "Eyesight to the Blind".
He is also mentioned in "The Real Frank Zappa Book" (1989): "Don was also an R&B fiend, so I'd bring my 45s over and we'd listen for hours on end to obscure hits by the Howlin' Wolf, Muddy Waters, Sonny Boy Williamson, Guitar Slim, Johnny "Guitar" Watson, Clarence "Gatemouth" Brown, Don & Dewey, the Spaniels, the Nutmegs, the Paragons, the Orchids, the etc., etc., etc."