They use different techniques today, but those old guns were electroplated. Removing the nickel involves simply reversing the process and that can be done by any good plating shop. (Most of them deal mainly with dinner ware, trophys and the like, so talk to them first; don't just waltz in with a gun.)
Once the nickel plating is gone, you polish the gun in the normal way. With a sideplate gun, you polish the gun with the sideplate on so there is no gap. Polish with a hard wheel or by hand using a sanding block to avoid dishing holes. The result won't fool an expert, but those guns of mine turned out pretty good. Copies are available of most old grips, since so many were made of hard rubber (gutta percha) and are broken or cracked.
The bluing is regular caustic blue (tank blue). Altering the polishing method can give anything from a soft blue to a hard black blue more like the original.