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.