Launched in 2016 in Seoul, South Korea, BtoB Blue is a sub-unit of the K-pop boy band BtoB, which formed four years earlier. BtoB enjoyed success right out of the gate with the group's debut album, Complete, which charted at Number 1 in 2015. Although the group's lineup featured a combination of rappers and singers, BtoB took a different approach when forming the sub-unit BtoB Blue, whose roster focused on the melodic sound of vocalists Eunkwang, Changsub, Hyunsik, and Sungjae. BtoB Blue's four members appeared on the television show Immortal Songs: Singing the Legend in 2016, ultimately winning the singing competition with a performance of "To My Love." Later that year, the group officially debuted with the single "Stand By Me," which climbed to number 15 on South Korea's Gaon Weekly Digital Chart and eventually went gold. BtoB Blue made another televised appearance in 2017, when the group competed in Immortal Songs 2. This time, however, the group lost the competition. One year later, BtoB Blue released "When It Rains" as the follow-up single to "Stand By Me." This, too, proved to be less successful than its predecessor, with the song stalled at number 58 on the South Korean charts. This marked the end of BtoB Blue's run, although all four members continued to perform with BtoB, enjoying Top 40 success at home as well as in Japan.