(1) A 12-bar form built on the I, IV, and V chords; (2 ) a scale with a flatted third, fifth, and perhaps a seventh; (3) a poetic form; (4) a way of articulating tones; (5) a set of verbal sentiments similar to those used in folk blues songs; (6) a vaguely defined, mythic "feeling" that some say is basic to all jazz.
The blues are an African American form, derived in part from the central Sudanic area of Africa (Mali, northern Ghana, northern Nigeria, and central Cameroon), and adapted to African American musical tastes and later to those of Euro-Americans. The historical sources of the blues have been debated for years, but evidence increasingly points to the Sudan as a likely point of origin. Whatever the specific sources, African musicians such as Toumani Diabaté and Ali Farka Touré have claimed the blues as a distant relative and have recorded with American blues musicians.
Just as popular songs have grown simpler in form over the years, so have the blues, losing much of their length, their varied structure, and their rhythmic complexity. Some writers on the blues, apparently unaware of the enormous body of country and folk blues, speak of a "refinement" in the blues of the city and vaudeville, a development that might better be described as standardization and simplification. In blues of the 1920s and 30s the form was far more flexible and irregular, and came in many forms such as chanted repetitions, elongated cries, and talking blues, all of which varied in bar length.
The melodies of blues are often described as characterized by "blue notes"—two or three flatted notes in a scale—but blue notes might be better described not so much as fixed pitches in a scale, but as areas around pitches that can be lowered or raised, or passed through on the way to a particular pitch. Some musicians speak of a "blues scale," with a "blue" or flatted third, fifth, and possibly seventh step of the scale, implying that the blues scale is a major scale that has been altered. It might, however, be better to think of blue notes as being tones that are bent or changed to increase the expressivity of the music, not merely to alter an existing scale.
Blues do not have to be sad, and not all sad songs (like "Stormy Weather") are blues. Many songs with "blues" in the title (such as "Blues in the Night") are in popular song form. The blues can be in major or minor (or other modes), and can be light-hearted, or even rhythmically driving (as played by the Count Basie band, for example). They can even be played as a march (as in Art Blakey's "Blues March").
Skip James, "Devil Got My Woman" (1966)
Count Basie & Oscar Peterson
Art Blakey & the Jazz Messengers, "Blues March"