Pokémon Blue was initially sold in Japan as a CoroCoro Comic mail-order special edition.
The Japanese Blue version followed Pokémon Red and Green, which had launched in February 1996. It was offered to CoroCoro Comic subscribers on October 15, 1996, making it a special early release rather than an immediate general-retail game.
Blue featured updated in-game artwork and revised dialogue. Its code, script, and artwork later became the basis for the international Pokémon Red and Blue releases, which explains why overseas players often saw a different presentation from the original Japanese Red and Green games.
Japan eventually received Pokémon Blue at general retail on October 10, 1999. Pokémon Yellow was a separate enhanced version inspired partly by the anime, while Pokémon Green was one of the original Japanese launch versions, so neither is the answer to this mail-order question.