I will take a stab at it and say black oxide, which is in fact a form of bluing. Now then it seems to me that you might be able to get a black finish by using cold bluing over and over. Might be worth a shot to Email Mec and ask them. I remember years ago I had a rough looking machinist's height gauge redone on the company's dime. I worked for Valenite Metals at the time and they had in house black oxide processes to put a finish on milling cutter bodies and components. Might look around for a gunsmith that does hot bluing and ask them about a black finish. Some of newer type bake on gun finishing systems might just work also. Check out Brownell's or Midway USA. good luck Bill. PS let us know what you find out.