I do not have a lot of experience trying to clean up 100 year old soaked wood. Was my impression that the "black" might be a combination of dirt, sweat and linseed oil that has been oxidizing with oxygen from the air. I mostly have used Circa 1850 Paint and Varnish remover - no sanding or scraping with metal scraper. Is a couple long stocks here for P14 that are nearly all black - no evidence at all that they were made that way - just 100 years plus of the air reacting with that linseed oil, I think.
I did use that Circa 1850 stuff on a WWII re-build M1917 - that long stock and the hand guards had numerous dents and dings - many "dark" or "black" marks - I was pleasantly surprised that most of that crap came off nicely - and I discovered some "brands" or "cartouche" in that wood, that I did not know was there, under that skin of crap. From what I had read about USA re-build - is probable / possible that the long stock might have been made in WWI, and at least one of the hand guards might have been made during WWII. Taught me not to use sandpaper or scraping - I would have removed those stamps without knowing they were there.
For better or worse, on some Swede military stocks - I set on a tray and slathered on acetone - liberally - in an attempt to soften up visible "grease" and try to pull some of the oils out of the wood. Followed with really "hard" rubbing with terry cloth rags - seemed to pull out stuff. I had read to apply acetone and then lay on layers of newsprint or similar - to draw that oil out of the wood. To some extent, is likely defeating the point - I think old school military finishes were meant to be soaked INTO the wood grains - not sitting in a layer ON TOP of the wood.
Contrarily - I am not so sure that gun "lube" and oils were meant to be soaked into the wood - several instances of recoil lugs and inletting gone "punky" - I am sure is from the "wrong" oils going into the wood - so is value, I think, to get those out of there. Not sure that most military guns put together with notion to still be used 100 years later - I suspect projected time of a war-time soldier's use was in weeks, not years.