I was asked to darken a Pederosoli Brown Bess that had 'European hardwood", basically a birch. I used aqua fortis, followed up with black oil artist paint that I rubbed into the stock by hand. I let it set for a week, applied dark brown leather dye to take the edge off the harshness of the black (it helped reduce the opaqueness of the paint). I then rubbed back certain areas to give an aged appearence and oiled it up. It looks like old black walnut.