This blade was etched in acid (the same I'd use for damascus steel) and then the spine was polished back up. The recessed areas are a dull gray. If the entire blade was hard (the spine on this one was left soft) the recessed areas would be black.
You can etch the the blade lightly with any of the typical treatments (gun blueing/browning, archer's etchant (FeCl)) and then polish the entire blade except the recess.
Another option is to bring the recessed areas to a mirror polish, then go back and put a satin finish on the rest of the blade.
I've seen guys use black lacquer as well.
If you do the filework before hardening and tempering the recessed areas will be black. Just don't touch them after that and they'll stay that way.