"music form featuring flatted thirds and sevenths," possibly c. 1895 (though officially 1912, in W.C. Handy's "Memphis Blues"). Blue note "minor interval where a major would be expected" is attested from 1919, and at first was suspected as a source of the term.
I am under the impression that these terms [blue note, blue chord] were contemporary with, if they did not precede and foreshadow, the period of our innumerable musical 'Blues.' What the uninitiated tried to define by that homely appellation was, perhaps, an indistinct association of the minor mode and dyspeptic intonation with poor digestion; in reality, it is the advent in popular music of something which the textbooks call ambiguous chords, altered notes, extraneous modulation, and deceptive cadence. [Carl Engel, "Jazz: A Musical Discussion," The Atlantic Monthly, August 1922]
"depression, low spirits," 1741, from blue (adj.1) in the sense "low-spirited" (c. 1400).