I hesitate to add to this as I don't have a whole lot of experience, but I had good success darkening a forearm by using a tip I found in James V. Howe's book. It involves fuming the wood with ammonia. I stood the forearm up next to a saucer full of household ammonia and turned a plastic trash can upside down over it and left it overnight. Next morning it was quite a bit darker and the finish was not harmed. I was happy!

Regards, Joe