Meaning

noun a type of folksong that originated among Black Americans at the beginning of the 20th century; has a melancholy sound from repeated use of blue notes

noun a state of depression

noun blue color or pigment; resembling the color of the clear sky in the daytime

originIn reference to the ancient Roman and medieval Byzantine racing faction, a calque of Latin venetī (“Sea-Blues”) or factio veneta (“the sea-blue faction”) and Byzantine Greek Βένετοι (Bénetoi, “Blues”).

Etymology from Wiktionary, CC BY-SA 3.0