Pokémon - The Complete Story - Vol. 1 Ch. 7

[MangaDex]

 

[Midoriha]

That even so back the idea that Blaine was part of Mewtwo's creation was a thing is really interesting.

 

[BladeZX]

That even so back the idea that Blaine was part of Mewtwo's creation was a thing is really interesting.

That backstory was also used in the special/adventure manga, but since this is supposed to be closer to the game maybe originally Blaine was Mewtwo's creator.