"Blue Moon" is a classic popular song. It was written by Richard Rodgers and Lorenz Hart in 1934, and has become a standard ballad.

The lyrics presumably refer to an English idiomatic expression: "once in a blue moon" means 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 (or she) finds love.

Richard Rodgers and Lorenz Hart were contracted to Metro-Goldwyn-Mayer in May 1933. They were soon commissioned to write the songs for Hollywood Party, a film that was to star many of the studio's top artists. Richard Rodgers later recalled "One of our ideas was to include a scene in which Jean Harlow is shown as an innocent young girl saying - or rather singing - her prayers. How the sequence fitted into the movie I haven't the foggiest notion, but the purpose was to express Harlow's overwhelming ambition to become a movie star ('Oh Lord, if you're not busy up there,/I ask for help with a prayer/So please don't give me the air...')." The song was not even recorded and MGM Song #225 "Prayer (Oh Lord, make me a movie star)" dated June 14, 1933, was registered for copyright as an unpublished work on July 10, 1933.

Lorenz Hart wrote new lyrics for the tune to create a title song for the 1934 film Manhattan Melodrama: "Act One:/You gulp your coffee and run;/Into the subway you crowd./Don’t breathe, it isn’t allowed". The song, which was also titled It's Just That Kind Of Play, was cut from the film before release, and registered for copyright as an unpublished work on March 30, 1934. The studio then asked for a nightclub number for the film. Rodgers still liked the melody so Hart wrote a third lyric: The Bad In Every Man, (Oh, Lord …/I could be good to a lover,/But then I always discover/The bad in ev’ry man), which was sung by Shirley Ross made up in blackface. The song, which was also released as sheet music, was not a hit.

After the film was released by MGM, Jack Robbins — the head of the studio's publishing company—decided that the tune was suited to commercial release but needed more romantic lyrics and a punchier title. Hart was initially reluctant to write yet another lyric but he was persuaded. The result was "Blue moon/you saw me standing alone/without a dream in my heart/without a love of my own".

In actual fact, there is another verse that comes before the usual start of the song. Both Eric Clapton & Rod Stewart used it in their recent versions of the song. The last line of this extra verse is "Life was a bitter cup for the saddest of all men."

Robbins licensed the song to Hollywood Hotel, a radio program that used it as the theme. On January 15, 1935, Connee Boswell recorded it for Brunswick Records. It subsequently was featured in at least seven more MGM films including the Marx Brothers' At the Circusand Viva Las Vegas. Part of the song was in the musical Grease.

Since 1934, the song has been recorded by many performers. A partial list follows:

Mel Tormé's version was the only one that actually reached the Billboard magazine charts; it was released by Capitol Records as catalog number 15428. It first reached the Billboard magazine Best Seller chart on April 8, 1949 and lasted 5 weeks on the chart, peaking at #20. The record was a two-sided hit, as the flip side "Again" also charted.

The first crossover recording to rock and roll came from Elvis Presley, but the version that really stirred things up came from The Marcels, a doo-wop group. In 1961 the Marcels had 3 songs to record and needed one more. Producer Stu Phillips did not like any of the other songs except one that had the same changes as Heart and Soul and Blue Moon. He asked them if they knew either, and one knew Blue Moon and taught it to the others, though with the bridge or release (middle section - "I heard somebody whisper...") wrong. The famous introduction to the song ("bomp-baba-bomp" and "dip-da-dip") was an excerpt of an original song that the group had in its act. The record reached #1 on the Billboard Pop chart for three weeks and #1 on the R&B charts. The Marcels version of "Blue Moon" sold a million copies, and is featured in The Rock and Roll Hall of Fame's 500 Songs that Shaped Rock and Roll.

In 1967, Eric Clapton used a portion of the song in his guitar solo from Cream's hit "Sunshine of Your Love." Bob Dylan covered the song on his Self Portraitalbum in 1970. In 1978, an arrangement by Jeff Funk was used in the film Grease. This has been followed by a country version from The Mavericks. More recently, it has been recorded by Rod Stewart. Elkie Brooks. Cybill Shepherd sang "Blue Moon" on an episode of Moonlighting(the detective agency in that show was called "Blue Moon Investigations".)

In Jim Jarmusch's 1989 film "Mystery Train", the three distinct stories that make up the narrative are linked by a portion of Elvis Presley's version of "Blue Moon" (as heard on a radio broadcast) and a subsequent offscreen gunshot, which are heard once during each story, revealing that the three stories occur simultaneously in real time.

A version suitable for performance by chamber choir arranged by David Blackwell is in the collection "In the Mood" published by Oxford University Press.