If you are good enough, you can ask him his price for bluing if the gun is fully disassembled by you. But note that you would take full responsibility if any parts are missing when you go to reassemble it.
Since I always want to control the finish on any gun I have reblued, I always dis-and re-assemble, and polish, any gun I take in for bluing. Of course, I only take in the parts that need to be blued. The gunsmith knows me and knows I won't give him a lot of static if something turns up missing.
Jim
Jim