The definition of a black hole describes a body that's information is hidden behind an event horizon since the information of an magnetic field and its effect upon its surrounding objects contradicts this eventuality it would appear that this magnetic field would be within the event horizon and have no causal effect on the stuff that surrounds it but u could question the nature of the horizon and its properties and the theory that surrounds it but I'm not the one to answer that.

The definition of a black hole describes a body that's information is hidden behind an event horizon if the black hole has a magnetic field the field must be limited to within the event horizon as otherwise the field could have a causal effect on events outside the horizon and violate the definition