So I thought I'd just make a thread for all the things I know about Cirno's origin and interesting coincidences.

- In 1958, Sergio Cirno Bissi painted "Female portrait in 18th-century costume". The girl in the portrait had light blue hair: https://www.catawiki.com/en/l/100799957
- In 1978, YMO released the song "Yellow Magic". The song in the following video sounds very similar to Cirno's theme at 29 seconds: https://www.youtube.com/watch?v=K3DSwl3gMTc&t=39s
- In 1982, Kunihiko Mitamura released the song "Omoide No Itoguruma". The part that sounds like Cirno's song starts at 23 seconds: https://www.youtube.com/watch?v=Re0K9dqGt9Y&t=23s
- In 1989, Orup released the song "Min mor sa till mej". In it, it talks about a "blue-eyed idiot". The song also contains a portion that sounds like Cirno's theme at 2:27: https://www.youtube.com/watch?v=QfH5usYFxSo&t=2m27s
- In a 1996 Gamest interview, artwork was shown of Marisu and Teresu. The pattern of Marisu's dress might have inspired Cirno's dress.
- In 1998, ZUN introduced a blue-haired girl named Mai, a character in Touhou: Mystic Square for the PC-98. She is found in the frozen fields within Makai. Her theme song and icy theme could be a reference to the ninth circle of Hell in the Divine Comedy (circle 9).
- Volume 4 of Sarai was released in 2001 and introduced the character "チルノ" (name only). Volume 6 came out shortly later in 2001 and showed the Cirno character for the first time. Cirno's original name was supposed to be "チルド" (Chirudo), but for whatever reason ZUN changed the ド to a ノ. Cirno's personality and association with being a "baka" was likely inspired by the Cirno character in Sarai.

If I'm missing anything, feel free to comment!