Actually you can do the black oxide finish yourself.
Caswells Blackener
7696 Route 31
Lyons New York
Check out the weapons guild forum in firearms finishes.
Kit costs about $80 but can be reused over and over. Frank

I'm not sure what you mean by "french gray plating" 

My understanding that french gray metal coloring is a stage of the case hardening process achieved by VERY carefully over-polishing a color case hardened surface with fine abrasives until all the colors are gone and a uniform gray is left.  The color is in someways similar to the color we see on naturally worn and faded case colors, BUT it it done deliberately and completely uniformly across the whole surface.    I know that Steve Durren and Glen Fewless have both done it that way.