Are they wear marks from where the slide touches the barrel in recoil or have you skun it up against a foreign object?
If they are operating wear marks, no point in touching them up, you will get them back whenever you shoot the gun.
If they are mars, I think Bruniton is a coating and about all you can do is paint them with a magic marker. Birchwood Casey sells one that is supposed to cover up marks on guns. Cold blue would color the steel but I don't know how it would blend.