Blues is a music genre based on the use of the blues chord progressions and the blue notes. Though several blues forms exist, the 12-bar blues chord progressions are the most frequently encountered. Blue notes are sung or played at a slightly lower pitch than that of the major scale for expressive purposes. Blues emerged at the end of the 19th century as an accessible form of self-expression in African-American communities of the United States from spirituals, work songs, field hollers, shouts and chants, and rhymed simple narrative ballads.

The use of blue notes and the prominence of call-and-response patterns in the music and lyrics are indicative of African influences. The blues influenced later American and Western popular music, as the blues form became a basic pattern of jazz, rhythm and blues, bluegrass and rock and roll. In the 1960s and 1970s, blues evolved into a hybrid form called blues rock. In the 1990s, punk blues appeared as an outgrowth of the blues rock and punk movements.

http://en.wikipedia.org/wiki/Blues

In jazz and blues, a blue note (also "worried" note) is a note sung or played at a slightly lower pitch than that of the major scale for expressive purposes. Typically the alteration is a semitone or less, but this varies among performers and genres. Country blues, in particular, features wide variations from the diatonic pitches with emotive blue-notes. Blue notes are often seen as akin to relative pitches found in traditional African work songs.

Blue notes (*): b3, (#4)/b5, b7

The blue notes are usually said to be flattened third, flattened fifth, and flattened seventh scale degrees. The flatted fifth is also known as the sharpened fourth. Though the blues scale has "an inherent minor tonality, it is commonly 'forced' over major-key chord changes, resulting in a distinctively dissonant conflict of tonalities" as well as the blue notes. A similar conflict occurs between the notes of the minor scale and the minor blues scale, as heard in songs such as "Why Don't You Do Right?".