Pokémon Blue was released in Japan as a mail-order-only special edition in 1996.
Japanese Pokémon Blue first appeared on October 15, 1996, for subscribers to CoroCoro Comic. It was later released to general retail in Japan on October 10, 1999. The version featured updated artwork and revised dialogue compared with the earlier Japanese Red and Green games.
The Japanese Blue version had an especially important legacy because its programming, artwork, and script formed the basis for the international Pokémon Red and Blue releases. The international games kept the distribution of Pokémon from the Japanese Red and Green pair while using Blue's improved presentation.
This history explains a common naming mix-up. International players often assume Pokémon Blue was simply the worldwide counterpart to Japanese Pokémon Green, but the development lineage is more complicated. Japan received Red and Green first, then Blue as a special edition; overseas players received Red and Blue in 1998.