Black holes aren't that much more dangerous than the objects that they originally were. They don't have super strong gravity necessarily, rather their gravity is the same as whatever they were before. The reason they are more dangerous and can suck in time, is that they are compressed so much that their surface is below the distance where the escape velocity is greater than the speed of light (event horizon). The earth could be compressed so much to become a black hole, but it would be very small, and would probably decay very quickly.
The most dangerous thing would probably be other entities moving around, and radiation.