You might want to check up on something called "the no hair conjecture". This conjecture has a part in it talking about the absence of magnetic fields in black holes. It says that when a star is nearly at its critical circumference, and beginning to form a black hole, its gravity sucks its magnetic field lines down onto its surface, plastering them there tightly. When the black hole is formed, the plastered down field lines are all inside its horizon. No field lines would then remain sticking out of the hole. So, what i am saying is when a magnatized star implodes to form a black hole, the hole might well be born with no magnetic field whatsoever.