If after soaking in acetone you still have a persistent black stain anywhere on the wood, you have a black water or iron stain.
This is a totally different animal. Use Oxalic Acid (trade name - wood bleach) to remove it. It can be found in some hardware stores and most of the better paint stores. Follow directions and more importantly, precautions.

Best, Dan

Dan May
www.classicgunstocks.com
www.millersingletrigger.com