TITLE: Black Hole Pole Shift
QUESTION [0 upvotes]: I know that planets and stars can experience a reversal of their magnetic poles, but can a black hole also have a pole shift? And if not, why not?

REPLY [1 votes]: In the case of planets and stars, their magnetic fields are generated by the dynamo effect caused by circulation of electrically conductive material in their cores. if those circulation patterns are perturbed, it's possible for the circulation direction to flip, thereby reversing the sense of the magnetic field generated by the dynamo. 
A simplified description of the source of the magnetic field in a black hole is that this magnetic field was once possessed by whatever it was that originally gave rise to the black hole, plus whatever magnetic fields were possessed by any matter which subsequently fell into the hole. So, for example, if a neutron star with a powerful magnetic field is pushed over the edge into black hole collapse, the black hole retains that field, and from the viewpoint of an external observer, those field lines terminate at the event horizon.
This means that from the viewpoint of an external observer, the magnetic field appears to be a property of the event horizon, as if it were a sort of membrane that possessed electromagnetic properties of its own. 
To answer your question, then, since the root causes of the magnetic field of a  planet with a fluid iron core that exhibits circulatory motion and that of a black hole with an external magnetic field are quite different, the answer is no- to flip the field of a black hole, it appears you would have to drop into it enough magnetized material (of opposite polarity) to cancel the field.
I invite the experts here to weigh in with their comments and corrections.
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