Not used it on a bow, works on apple and I've used it on an oak carving too. My thoughts are that the more recent the wood was felled, the stronger the reaction - green oak goes really really black, oak felled a long while (low moisture content/kiln dried) goes a more grey colour. Also, as it is absorbed into the grain, it is difficult to control so you can use it to colour in a large area but you can't use it to tint a small, precise section or easily get a sharp edge.
Incidentally, using bleach as well or instead of the vinegar is supposed to make a huge difference to the reaction - the end of this instractable is brilliant:
instructables.com/id/Steel-Wool-and-Vinegar-Wood-AgingEbonizingWeathe/
but make sure you read the conversation where an organic chemist joins in to explain everything, not the actual instructable itself (though it is useful)