No paint, no stains, no inks or dyes.
I used Richard Maguire’s ebonising method – take handful or two of oak shavings off the floor, boil for ten minutes in a saucepan, paint the resulting tea on the piece, sand down the raised grain, repeat 2-3 times. Then take steel wool, soak in vinegar for a week in a jar with a hole in the lid. Now paint the piece with that solution. The tannic acid in the oak, boosted by the tannic acid in the tea, reacts with the iron in the steel wool solution, and turns jet black.
It’s not a bad result, even on a quickly-done test piece that only got two doses of the tea and not much prep work.