Three chords are used, with a different chord area emphasized in each line of text. Often, each section is divided into two measures of sung text and two measures of instrumental fill like a comment on the words. Arising out of the melding of European and African concepts of pitch, they create a tension of being in between pitches of traditional European scales. The blues scale includes a flatted third, fifth, and seventh degree. Often there will be sliding to and through these notes.
Blues is a genre.
Blues is a genre, a whole category of music, with its own repertoire. With antecedents in other African American folk styles such as slave songs, levee hollers, ring shouts, spirituals, work songs, etc., blues is a genre with its own 100-year history of development.