The kitten formula of Revolution and Advantage are both safe to treat fleas in rabbits. You put the drops on the back of their neck. DO NOT use Frontline or any brand with the same drug as Frontline. Unless your rabbit is treated with an appropriate medicine, you will probably not get rid of the fleas. You really need to make sure that he is flea free, otherwise he could get sick or become anemic.
You also have to keep his area in the house very clean, with frequent vacuuming. Fleas only live on the animal at times and at other times they are in carpets and upholstery, where they can lay eggs. Then those eggs hatch and they can re-infest the animal.
The tiny black things you are seeing could be flea eggs, it could also be flea dirt, which is flea feces/dried blood. If you wet those black spots, they would likely turn red since they contain blood, so if they didn’t turn red, they might not be flea dirt.
A rabbit’s fur color can change, after a big shed or molting. A lot of times, different fur colors are mixed together on a rabbit, so maybe that’s why it looks like his fur is changing color.