What is the difference between Japanese Pokémon Blue and Western Blue?

Japanese Blue, the CoroCoro Comic mail-order edition of October 15, 1996, was a genuine third version: Game Freak redrew the Pokémon sprites, revised the script and remixed which Pokémon appear where. Western Blue, released in North America on September 28, 1998, inherited that engine and artwork but kept the encounter tables and version exclusives of Japanese Green.

The differences are concrete. In Japanese Blue, Jynx can only be obtained through an in-game trade, while in Red, Green and Western Blue it is caught wild in the Seafoam Islands. Japanese Blue's Game Corner prices differ too, with its Porygon costing 6,500 coins against 9,999 in Red and Western Blue and 8,300 in Green.

Even the dungeons moved. The unknown dungeon at Cerulean City, the cave that opens after the Elite Four, has a different layout in Japanese Blue from the layout in Red, Green and the Western pair, and the wild Pokémon inside differ between versions as well.

For a collector, the practical point is that a Japanese Blue cartridge is not a language variant of the Western game. It is a distinct edition with its own balance, which is exactly what a special edition sold only by magazine mail order was meant to be.