Like the other poster said, when an object turns into a black hole, all it is really doing is reducing it's radius to a very small point. So small that it's escape velocity is greater than the speed of light, therefore it turns black because light cannot travel fast enough to escape.
The gravity at the black hole's surface is greater than the speed of light, which makes it black.
If Jupiter was a black hole, it would be about 10 feet across. If earth was a black hole, it would fit in your pocket. But these objects would still have the same gravity. So the moon would stay in orbit.