Check with plating shops. Plating can normally be removed by reversing the electroplating process. Note though that since plating takes space (much more than bluing), parts had to be fitted to allow for the plating, so without the plating, parts may be loose or show gaps that were not there before.
Many plating shops will do this, but check first to see if they can do it and if they will work on a gun. They are not gun shops, so don't just waltz in with a gun. Even if they will tackle the job, they will probably require you to disassemble/reassemble the gun yourself and take responsbility for any breakage, etc.
If you want a black finish, you might discuss "black chrome", a process I know very little about but which gives a beautiful black color and would eliminate the fitting problem, since a new chrome surface would be applied.
Jim