The gravity is so strong that nothing can escape its pull, not even light, so it would appear black. The Earth's gravity is nothing compared to a black hole's. A rocket merely travels at 25,000 miles an hour to escape our planet's pull. That rocket could travel 671 million miles an hour, the speed of light, and still not escape from a black hole. Since nothing can travel faster than light, nothing can escape a black hole once it is in its clutches.
Black holes have a boundary of no return.
Once an object crosses the boundary, called the event horizon, the object is theoretically lost forever. Outside of the event horizon, or even close to the edge, an object still has a fighting chance to escape. The gravity is strong there, but light can get away. Within the event horizon, the game is over. All matter is infinitely stretched and, at the very center of the black hole, it is crushed to what physicists call a singularity.