Avoid solvents with ammonia, these usually say "Copper Remover" in bold type.
Nickel finish was the premium finish in the Black Powder cartridge days. The Army wanted blue for combat reason (and correctly so!) but the Cowboys on the Open Range were in the weather for prolonged periods and Nickel was/is more weather resistant. The preferred method for cleaning Black Powder fouling from any firearm is to use hot soapy water, and lots of elbow grease. (Use a Nylon bore brush and a soft tooth brush. It is more important to be gentle, you are allowed to clean it more than once to get stubborn fouling. Be sure to get under the ejector star!) If Nickel will hold up to all the sulfur in the powder and mercury in the old primers, it is fantastic stuff! All those old nickel guns with the flaking finish are from people improperly ?meaning, NOT) cleaning their gun, and or even worse using chemicals that are hard on the Electro Plated finish.
Polish gently with Flitz or similar product, All polishes have some form of fine abrasive (Many say they don't, but still be careful!)
Ivan