Msellers..."McClellan saddles were dyed using vinegar and iron filings to make them black. It might work on bone, would be a cool experiment anyways".
Be careful about using it on bone. The discoloration is based on the tanning process of leather. Bone is too dense.
When I was taught to dye this way my master shoemaker pointed out that the ring of black left around the nail hole was the result of the iron, not the vinegar and vinegar was only a penetration and spreading agent. It works best on cowhide, he told me, and is spotty on other leathers. We didn't get into bone because we weren't making corsettes.
Hungry Horse has some good ideas, and dye will work but you gotta..."read"...the bone to find out where the porous parts are. That's where the dye will go in deep. I found that putting it on well before the final polishing and then giving it a session on my brush wheel or some steel wool will get a good effect. Polishing the bone, or coating it with something like Permalyn will also help restrain the sun's effect of bleaching.
Never thought about why...
Capgun