It may take a little time, but if you have a glass beader available it will (should) remove the nickle finish. I would start with something rather aggressive, then go down to the finest available for a real satin finish. The gun will need to be completely disassembled, and each part glass beaded separately.
One thing to keep in mind, often when some of the older guns were found to have excessive pits in the finish they were plated to hide them, this may be the reason it was plated to begin with. The point, be prepared to purchase replacement parts if they are found to be pitted too bad to reuse, parts should be available either from Brownell or S&W.
I use the glass beading on all guns I re-blue. I no longer do the bluing, but do the preparation and then take it to a smith friend for the bluing process. If you do it yourself, keep in mind it is a SLOW process, get in a hurry and you can really mess it up. Be sure to plug the barrel and cylinders before starting the glass beading.
As for the Ruger, anodizing is likely the best choice for darken the gun, but it too can look new with a matte finish by running it through the beading process. It won't be dark, but should have a nice matte finish.
Good luck...........