In jazz and blues, a blue note is a note that—for expressive purposes—is sung or played at a slightly different pitch from standard. Typically the alteration is between a quartertone and a semitone, but this varies depending on the musical context.
Origins and meaning
Like the blues in general, the blue notes can mean many things. One quality that they all have in common, however, is that they are [[flat (music)
The blue notes are usually said to be the lowered third, lowered fifth, and lowered seventh scale degrees. The lowered fifth is also known as the raised 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". 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?", "Happy" and "Sweet About Me".
In the case of the lowered third over the root (or the lowered seventh over the dominant), the resulting chord is a neutral mixed third chord.
Blue notes are used in many blues songs, in jazz, and in conventional popular songs with a "blue" feeling, such as Harold Arlen's "Stormy Weather". Blue notes are also prevalent in English folk music. Bent or "blue notes", called in Ireland "long notes", play a vital part in Irish music.