blue note

In jazz and blues, blue notes are notes sung or played at a lower pitch than those of the major scale for expressive purposes. Typically the alteration is a semitone or less, but this varies among performers. Country blues, in particular, features wide variations from the tonic but still with the blue-note feeling.
The blue notes are usually said to be flattened third, flattened fifth, and flattened seventh scale degrees, although they approximate pitches found in African work songs. These blue notes are what turns a major scale into the blues scale. The same transformation of notes transforms the minor scale into the minor blues scale, as heard in songs such as "Why Don't You Do Right?".
The blues scale is used in almost all twelve-bar and eight-bar blues, but it is also used in blues ballads and in conventional popular songs with a "blue" feeling, such as Harold Arlen's "Stormy Weather".
In its earliest manifestations, the flattened third, or mediant, and flattened seventh, or subtonic, were the main blue notes.
Blue notes are also heard in English folk music (Lloyd 1967, p.52-4), but are not usually in the usual blues progression