The finish is a simulated "Case hardened" look.
Originally, in the 19th century, the steel was case hardened, this is a process where low carbon steel would be machined in it's soft or unhardened state. Once the machining was done, the steel part would be packed in bone meal or another source of carbon and then placed in a heat treating vessel (much like a kiln). Heat would be applied and the carbon from the bone meal would migrate into the interstitial spaces in the steel crystals.
This would harden the outside or "Case" of the steel but leave the inner part ductile and tough. A very effective and simple method of making a steel part hard on the outside (to guard against abrasive wear) yet remain tough and ductile so it wouldn't be brittle and break. Modern firearms are made from much better steel than was available in the 19th century and case hardening is no longer required.
I have this finish on my Vaquero and it has held up beautifully since 1994, no peeling or wearing off anywhere on the gun.
I expect my latest purchase to arrive next week (Chapparral Arms 1873 SAA), it also has the simulated case hardened look.
I hope it is as nice and durable as that of the Rugers.