Rifleball36,
This is not an uncommon finish. A bit time consuming, but well worth it.
The steel is browned, in a sweat box, WAY beyond what you would normally let it go, with NO carding between applications.
Then when it looks really cruddy, wirebrush, and sand all the browning off.
If it has gone long enough, there will be a good amount of pitting.
After it is cleaned off, it is then blued with a commercial cold blue. This is followed by steel wooling the bluing off with Breakfree CLP. This brings the bluing down, leaves the pits very black, and brings the highlights out to a nice dull gray. It also stops the browning and lubes the steel.
A lot of work, but a really nice durable finish.