"Blue" is a song by American rock band A Perfect Circle. It was the third single from the band's second studio album, Thirteenth Step, behind the releases of the singles "Weak and Powerless" and "The Outsider". While not as successful as the prior two singles, which crossed over into the Billboard Hot 100 charts and hit the top 5 of the Mainstream Rock charts, the band was still a moderate success, peaking at number 19 on the latter chart. A remix version, "Blue (Bird Shake mix)", was created by guitarist James Iha and released on the band's compilation album Amotion as well, and also received radio airplay.

After the release of the band's first album, Mer de Noms, lead singer Maynard James Keenan returned to his other band, Tool in late 2000. While Keenan and guitarist Billy Howerdel had originally planned on working on material for a second album, which would become Thirteenth Step, while he was on the road with Tool. However, Keenan found it difficult to try to juggle two bands at once, instead leaving Howerdel to work on the album with the rest of the band across 2001 and 2002. The song originates from Howerdel's early sessions for the album, being written shortly after seeing the effects of the 9/11 attacks in September 2011. Influenced by the dark scenes, Howerdel wrote thinking of themes of "escape" and "fearing the worst". The song's first iteration was written and recorded all in one take by Howerdel with a bass guitar, under the working title of "Red". After Keenan reconvened with the band for the album's final recording sessions over the first half of 2003, tension occurred between Keenan and Howerdel, due to Keenan wanting to rearrange and mellow out many of Howerdel's early sessions recordings such as "Blue". During this time, the song was given lyrics by Keenan, who tied its meaning more into the Thirteenth Step's concept of addiction and recovery, with the lyrics being about having to confront the aftermath of a person overdosing on drugs.

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