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.
The existence of the blue note within African-American music ultimately derives from the fact that the equal temperament utilised in western diatonic harmony is an artifice or compromise originally employed in the eighteenth century to address the problems posed in the creation of keyboard instruments. Equal temperament was an artificial 'straightening out' of a tendency for the natural harmonic series (musical intervals as they exist in nature) to go off at a tangent, meaning that higher intervals and octaves in their natural form are of a different pitch than the lower intervals and octaves. This made it difficult to create keyboard instruments which were 'coherent'. Hence the blue note is an attempt to correct this artifice by playing a note that is closer to the interval as it exists in the natural harmonic series. 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.