Q:

A magnetic field is produced by a circulating charge, so if the net charge is zero a black hole should not have any magnetic field, so how could a a black hole have a magnetic field to produce the jets seen rushing away from them?

A:

Although it is possible though not believed likely, that a black hole could be charged, they are not expected to have a magnetic field. The magnetic fields around a black holes that are thought to produce the spectacular jets of high-energy particles rushing away from black holes come from the disk of hot gas around the black hole, not the black hole itself.