I would think a convincing black "grunge" patina would need to be on top of the finish (thats how the originals get that way right?). Mike's process makes 100% sense to me.
On the flip side, I am just a novice with little experience, but I refinished a Lyman GPR a few years back and I think the wood at least turned out really nice (gun was sold and I don't have pics, sorry). It was a relatively open grain walnut.
Instead of going all out with a grunge patina, I simply used black india ink to fill the grain right before the whiskering and burnishing. I used a walnut stain first, let it dry, then I slathered the ink on, an it made the gun look gray like a black and white photo. Then when it was only partially dry (because I got nervous lol), I rubbed it back with the whisker/burnish process, so that it only darkened the pores. Its still looked very flat and dull until I applied the finish, The colors of the wood came out and looked very rich and convincing to me.