“BLUE” is the tenth and final song featured on Billie Eilish’s third studio album HIT ME HARD AND SOFT.

This song is a reworked version of two of Eilish’s previously unreleased songs. The first song is “True Blue”, which Eilish and her brother, producer, and co-collaborator Finneas O'Connell recorded when they were fourteen and eighteen respectively. The full version of the demo was later leaked in February 2022, inspiring Eilish to work on it again. The other song is “Born Blue”, which was going to be featured as track six on Eilish’s 2021 album Happier Than Ever, but Eilish “just couldn’t figure it out.”

The first half of the song, which is the reworked “True Blue,” takes the melodies and chorus, while adding a verse that reminisces each track from the album. The second half, which is the reworked “Born Blue,” covers somebody who grew up in a cold-shouldered family and their personality now that they’ve grown up.

At the end of the song, Eilish says, “But when can I hear the next one?” It is rumored but unconfirmed fan-theory that HIT ME HARD AND SOFT is going to be a double album, one revolving around the color blue, the other red. This theory is called the “Ilomilo Theory” because of the colors of the characters in the 2010 video game Ilomilo and how Eilish has a song of the same title.