If I wanted to produce a finish like that, I'd first use a blue remover like Birtwood Casey sells at wally world - if you heat the metal some first, it works faster.
Then, once it's entirely in the white, I'd fill a bucket with hot, high acid vinegar and then I'd put the pistol in - I'd plug the bore for and aft though with rubber stoppers, and have some oil in there in case they leaked.
Let soak overnight. Remove and lightly buff with 0000 steel wool. Should give a similar look.