The iron most commonly used in gun making was "wrought iron". There were many variations and names, including "(re)fined iron", "bloomery iron" , "malleable iron", and others. Colt revovlers were made of wrought iron into the 1890s and Winchesters into the 1870's. The main reason for case hardening was not decoration, as many suppose. Iron, unlike steel, cannot be heat treated for hardness, so in order to harden iron against the wear of internal parts, it was necessary to carburize or surface harden the metal. The color was a byproduct.
Many guns, like the previously mentioned Remington double derringer, were made of wrought iron to the end of produciton c. 1936. That is why broken hinges cannot be welded, but must be brazed. (Most of those hinges were broken, not by firing, but by ignorant users flipping the gun open.)