Understanding the Jazz Blues
The birth of the blues is shrouded in mystery and uncertainty, the music thought to have developed gradually over the course of several decades at the end of the 19th century. Amateur historians and academics alike agree that the blues is an amalgam of various musical styles, from West African Griots and the work songs and field hollers sung by primarily African slaves to Appalachian folk music, ragtime, and early jug band music. American songwriter and jazz/blues great W.C. Handy is said to have heard music closely resembling the blues as early as 1892, and Handy wrote several blues songs like "Memphis Blues" and "St. Louis Blues" that would become jazz, rather than blues standards. The blues experienced its "coming out" party in 1920 with what is widely considered to be the first true blues song recorded, Mamie Smith's "Crazy Blues."
During the late 30s and into the Bebop era, Jazz Musicians started to play more complex versions of the blues. They added chords to enhance the cadences to the important chords and thus creating more scope for improvisation. Mostly this meant adding II-V-I progressions.