Pokémon Blue was the enhanced Japanese game adapted into the international Pokémon Red and Blue releases.
In Japan, the first Pokémon games were Red and Green, followed by an improved Blue version. Blue revised elements such as graphics, encounter tables, and aspects of the game’s presentation. International Red and Blue were based on this enhanced version rather than directly on the original Japanese Red and Green code.
The international games still used the Red-and-Blue branding and retained version-exclusive Pokémon, so players needed to trade between versions to complete the Pokédex. The naming difference can cause confusion because Japan’s original pair was Red and Green, while most overseas players first knew the series as Red and Blue.
Pokémon Yellow later offered another enhanced experience, adding Pikachu as the player’s starting partner and incorporating more elements associated with the anime.