Depends on what you want to do with the fret board as I reckon the rose wood goes well and much better than the alternative that could have been maple. If you apply some lemon oil or fret board wax as a sealer they will both darken the timber a bit but the only way to permanently make it darker is to apply black ink but that would also require a layer of something over the top to seal it in. More work and probably not worth the effort when you consider how many rose wood fret boards are out there.
With the back of the neck are you dry sanding? If so have you noticed whether the old finish is clogging up the grit paper? Wet sanding with 400 should strip it back in no time at all and remember to keep rinsing the paper in water with a small drop of dishwashing liquid to help remove any build up in the grit.