Each of the characters in the manga were named after a Pokémon cartridge
In Japan, the games were first released as Red and Green, with Blue coming later as the updated third edition. Yellow was a variant of the game that was supposed to follow the story of the anime.
When it went overseas, instead of releasing the original games, they split Pokemon Blue in order to create the international versions of Red and Blue and they changed the male rival's name to Blue to keep it consistent with the cartridges
Because of this, the characters of Blue and Green traded names overseas in the Manga