Well, he's obviously a Dark type, as noted in the name Darkrai, we've got that, but being just a ghost type wouldn't really contribute anything new; they added in quite a few new ghosts in sinnoh, including a legendary Dragon. It wouldn't be as good to add in several of the same type, save for ones like Flying, Dragon, and psychic legendaries, which are open to more diversity.
Pure typing wouldn't really work for him, and we definitely don't want to make him a Ghost/Dark type. A legendary pokemon with no weaknesses and is capable of inducing sleep with a complimentary ability?
There are plenty of pokemon you could make a case of a different typing for. It might not make perfect sense, but not everything in pokemon does.