Here is a link to information about bluing or "black oxide" it's purpose and uses on guns. Hope this answers your question. you will have to copy and paste the link. http://en.wikipedia.org/wiki/Bluing_%28steel%29
Send it back to the factory or a trained professional
The reblue, even if factory, destroyed any collector value. As a shooter, it's worth about 100-300 USD, depending on how good the reblue is.
Retention of the firearm, and access to it.
The invention of the Firearm
For which make & model rifle??
The invention of the Firearm
Determines the use of the firearm.
Treat all weapons as if they are loaded.
Would have to see how much damage was done with the reblue. If this was a restoration, instead of a reblue, the gun could still bring over $1000. If it is a typically wheel-buffed hot-salt blue - which is totally inappropriate for that gun - value is maybe $400.
A 38 Special cartridge can generally be safely fired in a firearm chambered for 357 Magnum due to their similar dimensions, but it is important to consult the firearm's manufacturer or a gunsmith to ensure compatibility and safety.
200-300 USD. Maybe a little more if Colt did the reblue and you can prove it.
A shotgun is a firearm. A revolver is a firearm. A pistol is a firearm. A rifle is a firearm. If you want even more specific -- A Remington 1187 is a firearm (shotgun). A Smith and Wesson 686 is a firearm (revolver). A Sig Sauer P226 is a firearm (pistol). A Colt 6920 is a firearm (rifle).