Nitride is not a typical coating type of finish, its a metal treatment that leaves a harder surface layer of the metal it was used on. The surface actually becomes harder and more wear/scratch resistant. When used on stainless it also reduces the "shine" to a nice matte finish. Glock has been using a similar process on their handguns for ages.
Its about the least expensive way to get a "black stainless" product that lasts. Sure you could have a regular ss barrel BlackIced but it would end up costing more and would not be quite as durable. The Nitride treatment on the CVAs is barrel ONLY. The rest of the blacked out parts are just a typical coating of some type. The coating they use on the frame is known to wear and flake off. Mainly around the firing pin bushing.
The cost difference is so minimal why not get the better option? Online the price difference is as little as $15 at places like muzzle-loaders.com and MSRP price difference is around $40.