Why would you need to reblue the action after a feed rail modification? It's the bottom of the rail. I have done them myself with a drill and a grinding stone.
Why reblue, because if I'm paying someone to do this kind of work on my rifle I want it to come back looking like a new rifle. I don't like pulling my action out of the stock all the time (especially if if the rifle is shooing very well) when I clean to make sure rust hasn't started on the metal I exposed. Bluing protects metal and it doesn't take long for rust to get started even in a dry climate let alone a wet and humid one on metal in the white.