n.pl.
1. (prec. by the) a bout of depression (had a fit of the blues).
2 a (prec. by the; often treated as sing.) melancholic music of Black American folk origin, often in a twelve-bar sequence (always singing the blues). b (pl. same) (as sing.) a piece of such music (the band played a blues).
Derivatives:
bluesy adj. (in sense 2).