Blue note
In jazz and blues music, blue notes are notes added to the major scale for expressive quality. They correspond approximately to the flatted third, flatted fifth, and flatted seventh scale degrees, although they approximate non-chromatic pitches found in African work songs, specifically, the flatted seventh is often a justly tuned minor seventh. Blue notes are the most important notes in the blues scale.
In its earliest manifestations, the flatted third and seventh were the main blue notes. Emphasis on the flatted fifth was an innovation in bebop in the 1940s.