Pokémon Blue was first released as a CoroCoro Comic mail-order special edition.
In Japan, Pocket Monsters Blue was released on October 15, 1996, several months after Pokémon Red and Green. It was initially available by mail order to subscribers of CoroCoro Comic, a Japanese manga magazine, before receiving a wider retail release much later.
Blue featured updated in-game artwork and revised dialogue. Its code, script, and artwork were subsequently used for the international Pokémon Red and Blue releases, which replaced the Japanese Green version in overseas markets.
This history creates a common naming mix-up. Japanese Pokémon began with Red and Green, while many international players encountered Red and Blue as the first pair. Japanese Blue was not the same launch product as the international Blue branding, although it supplied important material for those overseas versions.