Here is the product professional engravers use to achieve the look you're after, it's called Background Antique and Vigor and is produced by Grobet USA and costs <$20 for the kit. One place to purchase this product is here:
http://ngraver.com/
It's very easy to use, with a small artist's brush you simply cover the entire engraved surface with the Background Antique, let it dry for 5-7 minutes and then with the Vigor (aul solvent) dabbed onto a swatch of lint free cloth you rub over the surface gently to remove the Background Antique from the higher areas.
Below the Background Antique and Vigor pic are pics of my custom Fox which has a French Gray finish with the treatment applied.
Hybrid Mode