How they work
We know basically how black holes are generated and what they are, so to nullify a black hole we generate a containment field around the 
anomaly, but in actual fact, we are containing the rest of the universe from the anomaly using a gravitational nullifying field. By generating a field of null-gravitational effect just outside the event horizon
with a gravitational pull resistant structure, we are able to stop the growth of the anomaly and therefore contain it. We can’t divulge the null gravitational field specifics but summarily everything that has mass has gravitational pull, the less mass, the less gravitational pull, so to create negative gravitational field we go beyond small mass, into nothingness. The generator pretty much makes a non-existent field. (This is why you don’t see anything when you operate the device)