That black stain is from a combination of solvents and powder residue. My nickel finish CVA Staghorn has the same stain in the breech. It does not hurt the integrity of the rifle. It just makes then look bad. There is nothing wrong with your Firebolt rifle. If it served you well for several years, my guess is it will continue to do so. Although there is nothing wrong with another rifle. And if you want one, then you should have one.

Nickel is nothing more then a blued barrel with a finish. Stainless is just that, stainless. They are a nice barrel. I own a couple of them as well and really like them. Stainless will stain, just less. It will also rust if you abuse it and do not clean it. They are a nice barrel though.