Ok. A black hole could easily exist on Earth.
However, if it were even the size of the marble, then it would have to be Earth weight, which would indeed be bad. However, a very tiny one that needs to be seen with a very powerful electron microscope shouldn't be that dangerous, as it's event horizon would be very tiny indeed and it's gravity range small.
Black holes are, though they have considerably more gravity, subject to the same gravity laws as the rest of the universe. decreasing strength I think due to the inverse of the square of the distance. Hence a quasar that weighed 1 billion solar masses wouldn't hurt us on Earth. In fact, that's why we aren't being hurt by black holes, because the nearest one, that we know of, is over 1000 light years away.
There is something that puzzles me though. How can a supernova be seen if it's remnant collapses into a black hole? Dues the "black holeification" process take quite some time, because if not, then wouldn't the gravity pull in the matter and light from the supernova, hence making it unessable from Earth?
I know that light can escape outside of the event horizon. On the horizon itslef I think it would be trapped and, even if not pulled in, couldn't get it as it would be being pulled in at the same speed it's moving, hence going nowhere, except perhaps into the black hole later when it gains a bit more mass.