This is what I've typically seen used to "ebonize" a fretboard. I used it to dye a tan leather guitar strap jet black 30 years ago, and it's still jet black. It's alcohol based, and soaks well into the wood, so I can't see it wearing off quickly, but if it gets some light spots, you just put it on again. I've been using the Stewmac Fingerboard oil, which is some kind of thinned linseed oil with dryers, excellent stuff. It penetrates into the wood, dries quickly, and seals it up without leaving a thick surface coat, just looks like a freshly oiled board. I would think you could dye the board, then put the Stewmac oil over it to seal it in , or maybe even tint the Stewmac oil with Feibings? Science! Wear gloves! This stuff will dye any skin it gets on black, and it takes a while to wear off.
https://www.amazon.com/Fiebings-Lea...leather+dye&qid=1598696057&sr=8-5&tag=mh0b-20
Stewmac sells it too, it's the same dye with their label slapped on it. They mention that you need to get any oil off the board with naptha, and sand it so it uniformly absorbs dye, sounds like a PITA if your frets are already in.
Black Fingerboard Stain | stewmac.com
People have literally painted boards, but I can't see it working for long, any surface applied material has got to wear through eventually, and it will wear right where you expect it to, in the "money zone" between nut and the 5th fret or so. Some people have used black CA glue on fretless boards. That might soak in a little if you used a water thin version, sounds messy and nasty, but it might work. Good luck! Maybe there is some new miracle ebonizer I haven't heard of, and somebody will weigh in.