What is 「群青｛ぐんじょう｝」refering to in the context of youth?

Question

The word 「群青｛ぐんじょう｝」frequently appears in the literatures or music lyrics about youth life. I can't find any other meaning other than "Ultramarine" in the dictionary. What does it mean in this context?

Notable works:

1. 「群青」（music by YOASOBI)
2. 「群青戦記」（manga by Masaki Kasahara)
3. 「群青領域」（recent drama in NHK)
4. 「群青」(novel by Ayako Miyagi) which became 「群青 愛が沈んだ海の色」(2009 movie)
5. 群青讃歌 (music by Eve)
6. 群青レイン (music by Jin)

Answer 1

I can only guess because this seems strange to me, too. Maybe the color blue, or the character 青, makes one think of youth because of its connection with 青春, and the character 群 conjures up an image of friends bonding with each other or something like that. It seems to have little to do with the original meaning of ultramarine as a color pigment.

When I hear the word ぐんじょう, I only think of the color. However, when I see it written as 群青 and used in those contexts, I get that kind of image. It means nothing concrete to me.