According to music scholars Sylvaine Diouf and Gerhard Kubik, a lot of the vocal stylings used in blues singing actually have their roots in the music and recitations brought over to America by African Muslim slaves. Its a claim that may seem far-fetched at first, but its been estimated that anything from 7 to 30 percent of the African slaves who arrived in the US were Muslims, so its at least plausible that their music had some influence on the development of African-American music in general. And this mpeg played by Diouf at lectures (which juxtaposes the Muslim call to prayer with an early blues song called "Levee Camp Holler") does seem to suggest some pretty strong links... I'd be interested to hear what you think, though (via Metafilter)
(In addition to singing styles, Kubik claims that blues also inherited slide guitar and the general preference for stringed instruments from Islamic African music traditions.)