The Japanese Pokémon Blue version became the basis for the international Pokémon Red and Blue releases.
In Japan, Pokémon Red and Green were the first games, followed by a revised Blue version. When Nintendo prepared the games for international markets, the programming and artwork were based on the more modern Japanese Blue version.
The international releases were still named Pokémon Red and Blue and preserved the version-exclusive structure that encouraged players to trade. The Japanese Green version was not released outside Japan, which is why many international players initially assumed that Red and Blue were the original pair everywhere.
This distinction also explains several differences between regional releases. The overseas games combined the updated technical foundation of Japanese Blue with the Pokémon distribution associated with the original Red and Green pair. Pokémon Yellow later offered another enhanced version with stronger visual links to the animated series.