Black holes are so named because they exert such extraordinary gravity, that not even light can escape them. You'd expect them to be dark. But there are some black holes whose concentrated brightness outshines all the stars around them. Here's why.
Black holes are what happens when matter collapses into so small a space that it basically becomes a single point. The event horizon of a black hole is the point at which anything that goes in cannot get out again. Even light is caught and forced into the center of the hole, so that the entire universe goes dark in that region of space. If they're so dark, how do we find them so easily? It's true that many scientists deduce the existence of black holes by looking at the gravitational pull on stars nearby, but they can't search the movement of every star everywhere. In some cases, the best way to find a black hole is to look for regions of bright light. In fact, when scientists see a point of extreme brightness in a relatively compact region of space, one of their first thoughts is that they're looking at a black hole.
The darkest objects in the galaxy shine so brightly because they wreak havoc on everything around them, and they do so in a specific way. Imagine attaching each end of a rubber band to a different car, and then driving the cars in the same direction. Yes, the same direction. If the cars are both going the same speed, there's no problem. But suddenly the front car starts getting faster. If the front car is going twenty miles per hour, and the back one is only going ten, it doesn't take long for the rubber band to get stretched to the breaking point. This is the same thing that happens to objects approaching a black hole. The side of an object near to the black hole is being pulled faster than the far side of the object. This is true whether the object is plunging straight towards the black hole or whether it's in an orbit - since the closer side of the object will be orbiting faster than the outside, and it will get pulled into a spiral. The friction between the particles in the object is so great that it can heat the particles up hotter than most stars. And since a lot of material can orbit a black hole, there's a lot of fuel to keep it shining.
Black holes don't shine forever. They eventually go dark, and they do so as they get bigger. Remember that the rubber band only stretches if the cars at either end are going different speeds. A black hole can pull an object in with a massive gravitational force, but that won't make the object light up. The difference in gravitational force exerted on each side of the object is what matters. If a black hole is a mile wide, and the object in its grasp is also a mile wide, then far side of the object - when it gets close to the event horizon - is twice as far from the center of the black hole as the near side. There's a huge difference in the gravitational forces that each side of the object will feel. It will stretch, heat up, and shine. If the black hole is two million miles wide, and the object is still only a mile wide, then the far side is not appreciably any farther from the black hole's center than the near side. The difference in the gravitational force on each side is miniscule. It won't be ripped apart — or at least it won't be ripped apart until it's well within the event horizon where we can't see it. As black holes get bigger, they're more able to tip objects whole into their event horizon, and they go dark. It takes a while, but finally, they live up to their names.