I use photo copier toner powder.
You can buy small quantity replacement toner on line or from many office supply type stores. Comes in many colors but basic black is the old standby.
Fine as talcum powder,,maybe more so. I keep a small amt in a pill bottle so I don't risk dumping the larger container over.
Simply shake a bit right onto the wet finish as I'm applying it and rub it into the surface. It gets right into the grain and stays there as the finish drys.
I sometimes add a bit as I put a few last coats of very thin rubbed out oil on the wood. A very small amt goes a long way!
Looks good as it highlights the wood carvings too. I use it on restorations of cartridge guns too in the checkering when brushing in a coat of oil. It puts that slightly darkened aged look back into the newly recut checkering lines on a restoration.