You can buy gun blueing and browning solutions -- oxidizing solutions -- for refinishing firearms.

I've heard of the old-timers finishing muzzle-loaders by painting them with vinegar and wire-brushing off the rust, then oiling to get a uniform dark brown. You need to do something similar to bring out the pattern on pattern-welded blades.

They tell me the old mountain men didn't think a knife was any good until it was uniformly tarnished.

A blade that was cared for but not polished back in the old pre-stainless steel days would be some shade of brown, black or blue.

clint