
Pokémon Blue is the third Pocket Monster game to be released in Japan, on October 10th, 1996. While it does not feature any new Pokémon or storyline, it does however change which Pokémon are found in the wild, as well as in-game trades. It also revised the graphics, sounds, engine and script of the original Red and Green Pocket Monster games.
These revisions are then used in the localization of the American Pokémon Red and Blue version.
Other changes from Red & Green are into the game as well, but the only ones that carry over to American Red & Blue is the change in the catch rate of Pokémon and a redesign of the Unknown Cave.