Eve officially announced the album on August 20, 2024, revealing the title, cover art and the release date on X (formerly known as Twitter).

Following the album's release, Eve revealed the overall theming in an interview with Natalie:[3] Japanese: れだけ曲がある中で、どの曲にも共通するものってなんだろうとぼんやり考えていたんですが、まず真っ先に「青」は入れたいなという思いがありました。青は自分の好きな色でもありますが、空や海のような光と相性のいい色味だなと感じていて。一方で寂しさや憂い、悲しさみたいな、そういう“ブルー”な気持ちも表現できる。その二面性というか、どこか矛盾しているところが、人間みたいで魅力的だし、それは今回作ってきた19曲、どの楽曲にも言えるんじゃないかと。 [With all these songs, I was vaguely thinking about what they all have in common, and the first thing that came to mind was that I wanted to include the color "blue."

That duality, or the somewhat contradictory nature, is what makes it so attractive, like a human being, and I think that can be said of all of the 19 songs I've created this time.

The Animate-exclusive "collaboration CD" contained previously released collaboration tracks "Like The Flatwoods Monster" with Deu (フラットウッズのモンスターみたいに) and "Pray" with Hiroyuki Sawano, while the Tower Records Japan-exclusive "self cover CD" included the tracks "Glorious Day" featuring Hatsune Miku, and "Memento" (メメント) featuring Nqrse.

[6][7] Under Blue compiles a significant amount of singles, many with tie-ins to Japanese media including anime series, film and commercials.

Before its release, the track was used in a television commercial for Bourbon Corporation [ja], a Japanese confectionary company.

[18] On October 1, 2024, the Eve Arena Tour 2025 was announced in support of Under Blue, starting on July 12, 2025, in Nagoya, and concluding on August 24, 2025, in Kanagawa Prefecture.