S&W quoted me $200.00/blue and $260.00/nickel to refinish a Pre-#18 yesterday. I never thought of stripping a nickel gun and blueing it, but that could probably be done.
It can be done, but I don't know if the factory will do it or not; you will have to contact them.
The nickel has to be removed first, then comes all the steps to blue it. This will get pricey, but if it is something you really want go for it. I think you might also be prepared to find your barrel markings, etc., may be buffed out more than you might prefer. Good luck.
It should cost no more then a re-nickel. If S&W has the ability to strip and re-nickle a gun they should have no problem, or reason not to, blue a once nickeled gun.....