Pokémon Blue was the international counterpart used alongside Pokémon Red for the original Japanese Pokémon Green game.
Japan’s first pair was Pocket Monsters Red and Pocket Monsters Green. For overseas releases, Nintendo used Pokémon Red and Pokémon Blue instead. The international games were based on the updated Japanese Blue version, which had revised artwork, dialogue, and programming.
This localization choice is why many players remember Red and Blue as the original pair, even though Red and Green launched first in Japan. Pokémon Green remained a Japan-exclusive release for many years.
Pokémon Yellow later became a separate enhanced version, so it is not the international name for Green. Pokémon Silver belongs to the second generation and was released after the original Red, Green, and Blue games.