Those post-64 rifle receivers are made of siliceous steel (silicon content makes them more shock-resistant and unlikely to shatter). They can be:
* Plated (with either iron, black chrome, or black nickel).
* Color case hardened
* GunKote or DuraCoat (I prefer GunKote becasue it goes on so thin)
* Painted with Brownells' baking lacquer and baked.
Or just live with it. It just looks bad, it still works just fine.
Never try to educate someone who resists knowledge at all costs.
But what do I know?