What happens when you stack "unblockable" and "unblockable except by"?
No, it's still just unblockable. There are two restrictions. One will prevent it from being blocked by most things, while leaving artifact creatures and black creatures unrestricted. One won't let it be blocked at all. If nothing blocks it, both restrictions are satisfied, so that's allowed. If a black creature blocked it, one restriction would be violated, so that's not allowed.