And a bone nut will be better than a plastic (polymer) nut because . . .

Buffalo horn is sometimes black, but I haven't found any sources for the real thing in decades. There's always ebony - the choice of the violin family instruments for 500 years. I've used it on guitars.

I recently did a bone nut for a bass that had an original black (ahem) "polymer" nut. I finished the nut, painted it with a black sharpie, then shot a couple of coats of clear over it. Of course I didn't think to take a pic of the nut. Here is a crop from the whole thing.

Corian solid color counter top material (I ordered a sample from the MFG). LMI has black horn. And as mentioned StuMac sells black Tusq. I haven't used the Corian yet but know folks that do and highly recommend it for nuts.

As alternatives to dying bone.

Some times a black nut just works well for an instrument aesthetically. I built a walnut 6 string banjo (gitjo/banjitar), with ebony FB and head veneer that had to have a black nut that put me looking for options. I ordered a custom black satin banjo head from Remo to go with the theme. Cool looking instrument.

Peg head included a night scene with silver big dipper and northern lights in dyed curly maple. Fun build.

Seems like I remember seeing one of the regular suppliers that has water buffalo horn which is black.

Edit: LMII has it.

https://www.lmii.com/nuts-saddles/1517-nut-black-horn-2-1564-x-2764-x-1964.html

Some people use India ink to stain fretboards black, it might work on bone also. There are synthetics made specifically for guitar nuts that are black - Graft Tech makes some of their Tusq nuts in black. And apparently this is going on a resonator - frequently maple or ebony is used for saddles on reso, I see no reason why it wouldn't work on the other end of the string.

Buffalo horn is sometimes black, but I haven't found any sources for the real thing in decades. There's always ebony - the choice of the violin family instruments for 500 years. I've used it on guitars.

.... for just over eight HUNDRED dollars, you get a black PLASTIC nut. Forgive, I misspoke... the nut is "polymer."....

I am going to add one more comment before I crawl back into my hole. If you look at almost any modern mass produced guitar from almost any of the major manufacturers in almost any price range they will have a nut made out of some sort of man made material - lets call them all plastics. Martin, Taylor, yadda yadda yadda have micarta or melamine or "tusq" or graft tech or ..... You know why? One they work pretty well and two they can be molded into shape for pennies.

I just made a bone nut for a nice young woman with a nice medium priced classical guitar. It came with a "melamine" nut and saddle, she wanted bone. I made them but I also recorded the guitar before and after and I ran Alain Lambert's spectrum analysis software on waveforms of notes taken before and after the change. Guess what, she can hear a difference (after all she just wrote me a check), I can't and the software doesn't show it. Your mileage will vary

I just made a bone nut for a nice young woman with a nice medium priced classical guitar. It came with a "melamine" nut and saddle, she wanted bone. I made them but I also recorded the guitar before and after and I ran Alain Lambert's spectrum analysis software on waveforms of notes taken before and after the change. Guess what, she can hear a difference (after all she just wrote me a check), I can't and the software doesn't show it. Your mileage will vary

Dang it man! Don't be confusing us with science and facts!