I'm a proponent of Ebony nuts. They were regularly used on steel string guitars from 1900 until after WW2.
I see original Ebony nuts still in use for over 50 years, so the "wear" argument is bogus.
But that wasn't the OP's question, so everyone, it is time to do some experimenting!
There is probably some nasty chemical that will darken bone, but I'm not going there. I doubt there is an easy way to turn bone dark thru & thru, so I guess the path is to cut the nut, then darken it before finally setting it.
I was excited at the outcome of the wood-cooking thread, I've darkened some fret boards that were too light-colored for guitars.
No reason we can't come up with some hocus-pocus here.