Bessie Smith
Carl Van Vechten, restored by Adam Cuerden, Public domain, via Wikimedia Commons

The Blues originated in the American South after Emancipation, evolving from Spirituals and African American work songs. Common musical techniques included pitch-bending, call-and-response, and the use of "blue" notes.

Blue notes are tones outside the prevailing mode. These were eventually somewhat standardized into the "Blues scale," a hexatonic scale consisting of the tonic, minor third (♭3), subdominant, tritone (♯4), dominant, and minor subtonic (♭7).

The Blues became prominent in the early 20th century, and singers like Ma Rainey (1886 - 1939) and Bessie Smith (1894 - 1937) helped to popularize the genre on a wide scale.

Structurally, the Blues tended to fall into "12-bar Blues Form."

I	I	I	I
IV	IV	I	I
V	IV	I	I