A flattened third or seventh WordNet 3.0 & Open English WordNet

In jazz and blues, a blue note is a note that—for expressive purposes—is sung or played at a slightly different pitch from standard. Wikipedia

Musical note played or sung lower than the corresponding note on a major scale. Wikipedia Disambiguation

Music term; note that for expressive purposes is sung or played at a slightly different pitch than standard Wikidata

Any of the notes that produce the characteristic clashes with the underlying harmony associated with blues music. The flatted third, flatted fifth and flatted seventh can all be considered blue notes when played over major or dominant chord progressions, while only the flatted fifth will clash with minor chord progressions. Wiktionary