AIXI with a TM-based universal prior will always produce predictions about the black box, and predictions about the rest of the universe based on what the black box says, that are just as good as any prediction the human can come up with. After all, the human is in there somewhere. If you think of AIXI as embodying all computable ways of predicting the universe, rather than all computable models of the universe, you may begin to see that's not quite as narrow as you thought.