Are you trying to Ebonize another wood or darken the Ebony? I'm sure it would be fine for both. I have used alcohol based stains for that purpose. My only concern would be adding to much oil to the board. As for wearing off with time, since the Ebony is usually unfinished re-application is no problem. The alcohol stains I use last for a long time due to good solvent penetration. Stewmac sells Fiebings oil dye, which is leather dye, for that purpose and claims it's used by various manufacturers so I guess it should work fine.
It turns out that the Fiebing's "black oil dye" marketed by Stewmac is in fact Alcohol based also and the oil part refers to oil tanned leather. So it appears alcohol dyes are the standard for evening out the color in ebony and ebonizing rosewood and it is what I have been useing for years. I have never finished fretboards of ebony or rosewood with any type of building finish.
I tried something to blacken a fairly light rosewood fretboard many years ago, and it gave the board a rather iridescent appearance. I believe that it was India ink, but can't remember for sure now. I'll find a piece of scrap rosewood and see if it happens again. It's probably a good idea to try out new finishes initially on a scrap piece of the same type of wood that you want to finish.