"Blue Moon" is a classic popular song. It was written by Richard Rodgers and Lorenz Hart in 1934, and has become a standard ballad. In 1961 it became a doo-wop hit when recorded by The Marcels and later by Jan & Dean.
The lyric presumably refers to an English idiomatic expression: "once in a blue moon", meaning "very rarely". (The origin of the expression is unclear; see article "blue moon"). The narrator of the song is relating a stroke of luck so unlikely that it must have taken place under a blue moon. The title relies on a play on words, since blue is also the colour of melancholy, and indeed the narrator is sad and lonely until he finds love. The song is noted for its ending with the exaggerated baritone "blue moon".