I have seen some reviews of Gander Mountain's gunsmithing by some people on the Sig Forum. I would HIGHLY not recommend it.
CCR Refinishing does black ceracoat and duracoat surfaces. He usually takes 6-8 weeks, but everyone is satisfied with his work (google "CCR Refinishing" to find the link). I would NOT go thru Gander Mountain for that.
Honestly - If ya want the best black finish, I'd get Birdsong's Black T - If ya google Blact T, U will find all sorts of positive comments - He doesn't have a website, but I may be able to find the # if U are interested (U can PM me).