Safe for use on factory blued, stainless steel, chrome and nickel guns. It removes powder fouling, surface rust, tarnish and discoloration from the barrel and stubborn black rings from the face of the cylinder.
If it is discoloration from heat, which is something I have never seen on a chamber hood like that before, then you might be able to get it removed with brass brillo. I had a pistol scope that had some of the bluing burned from muzzle blast. (Another story for another day... Top Gear Top Tip of the Day, a revolver in 357 Maximum with a barrel of standard 6" length and a pistol scope near the muzzle is asking for trouble... Thank heavens for lifetime warranties with no questions asked.)
Anyways, the glass survived, but the bluing had been singed and left a black burn mark on the bare metal. Brass brillo and a little bit of thin oil plus some elbow grease, I had it back to bare metal with no further damage to the bluing or base metal.
That Gun Brite that PJ mentioned is probably less invasive.
Isn't this the area exposed through the ejection port. If so it might be discoloration or some type of oxidation from handling. If it bothers you don't sand it off with emery paper. Try some of the cleaning product that won't damage the steel should work fine. Even very fine steel wool. Then lightly oil. 1917