What is the gun's finish? Stainless steel? Blued? Parkerized?
If it IS NOT a stainless finish, you can go to wally-world and get some Birchwood Casey's Super Blue or Perma Blue, and that will darken in the scratch somewhat, and you can blend it in pretty well when you polish the blueing in with extra fine steel wool. Super blue is a darker, more powerful blue than Perma blue. Theres probably a lot better (and more expensive) products and methods out there. This method will cost you less than $5.00.
Also, is the scratch deep enough to cut into the metal? Or just a finish scratch?