So, a few things. Your fingerboard is finished. It has either been oiled, lacquered, or varnish. You could try to bleach it all day and be unsuccessful. You would need to remove the finish. Second thing. Rose wood is fairly porous, but hour fret board is most likely fairly smooth. This mean it was sealed with something before it was cleared. What ever it was sealed with, most likely a very thinned version of what it finished with, soaks into the wood to seal and stabilize. You may need to sand a lot. Third thing. You will need to use draino to beach the wood once the finish is removed. Rose wood, mahogany, and dark red brown woods similar in density will bleach to a yellowish blond hue. The draino will crystal ice on the wood when it is dry. Lightly sand off with steel wool. Wear rubber gloves and a mask. Use vinegar to neutralize it after you done sanding. Once dried the wood will be ready for stain and finish.
Now that you know what will work. I will say this. It may be easier to paint. I think I would probably remove the fret board All together and order a new one from stew Mac or Luthiers mercantile. To be lazy, or I might just make one out of maple. You can get templates from the same place or make your own with your guitar. Know this though. If you embark on replacing the fret board altogether. Get help. Hint” use steam to soften the glue. Good luck on whatever you decide. Knowledge is power.