General condition is important, the finish is rather complex, the barrel and pump tube are normally solid brass with a silver coloured nickel plating, then a black oxidised finish on top of the nickel.

If the black finish is all there and original, that would be unusual, removing the stock (one screw) MAY show you the original finish.

Many have been stripped back to bare brass, or just to nickel, beware of a painted finish hiding a mess underneath.