The blue note in blues scale
Blues scale is the pentatonic scale with one more note (added in the scale). This note is known as “Blue note” and it is the flattened fifth in the case of the minor pentatonic, or the flattened third in the case of the major pentatonic. See that the note that was added is the same in both scales; it is just to memorize the Blues minor scale and transmit this note to all the other Greek modes while doing the solo.
Learn more here: http://www.simplifyingtheory.com/blues-scale-blue-note/