It'll be weird if the black maid is the witch it means that she only wants the duke to be hers like he is not allowed to go out, he is feared by many friends, relatives, and girls. Its cool I guess
@Ruzand, If you read chapter 0, the duke had a dream in which he was told that the witch was very close to him, and so he went to Alice and thought maybe it was her, but decided not. So the idea was brought up and is still possible, but more importantly the last panel of that storyline foreshadowed that Rob was actually the witch which I think would be a lot more interesting personally.