Blue Note

In jazz and blues, a blue note (also "worried" note) is a note that—for expressive purposes—is sung or played at a slightly different pitch than standard. Typically the alteration is between a quartertone and a semitone, but this varies among performers and genres.

The existence of the blue note within music derives, in part, from the fact that equal temperament in western diatonic harmony is an artifice or compromise originally used in the eighteenth century to address 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 that were 'coherent'. Hence, the blue note attempts 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.

"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 lower than one would expect, classically speaking. But this flatness may take several forms. On the one hand, it may be a microtonal affair of a quarter-tone or so. Here one may speak of neutral intervals, neither major nor minor. On the other hand, the lowering may be by a full semitone--as it must be, of course, on keyboard instruments. It may involve a glide, either upward or downward. Again, this may be a microtonal, almost imperceptible affair, or it may be a slur between notes a semitone apart, so that there is actually not one blue note but two. A blue note may even be marked by a microtonal shake of a kind common in Oriental music. The degrees of the mode treated in this way are, in order of frequency, the third, seventh, fifth, and sixth."

...
Wikipedia