More questions about the process:
Is it performed on new barrels, or broken in barrels?
Best to break the barrel in first. Any "rough" spots will become permanent after treatment. Kendricks recommend about 30 shots. At the same time, low round count barrels are recommended. Higher round count barrels have show a decrease in accuracy after being treated.
Would you do it on an new lapped custom barrel?
Definitely... especially a custom. A custom barrel, after chambering, crown, etc., is going to cost about $500-$700. The indications are that nitridng will increase barrel life 2-4 times. once again, best to break-in the barrel. Also, this process relieves stress in the steel and that may affect how your barrel shoots. Some factory barrels are not stress relieved, so there might be some difference in the way they shoot after treatment. Also, receivers may warp slightly after being treated, so best to bed the receiver after treatment. If it's already bedded, I would probably not get it done.
Is there a better wear for the bolt ways when a reciever is done?
You can have the bolt done as well. You need to remove any parts that might be damaged by the 100* heat first. Not all actions can be nitrided. Cast actions like Vanguard/Howa can not be treated. Rems and Savages can. Some customs can and some customs can't. It all depends on the steel that they are made from. Best to consult the treater first to make sure.
Black to Blue Black, satin to matte.
It sounds like surface, or case hardening.
It molecularly changes the surface of the metal.
There's some good info in this thread...
http://www.longrangehunting.com/forums/f19/what-do-yall-think-about-6-5-375-ruger-68438/
If you search the net, you'll find more info.