If quasars are powered by black holes, why are they so bright?

A black hole, in deep space is basically black, and very hard to detect. But if a black hole is surrounded by material, that material will fall towards the black hole and enter into orbit about it. (Black holes don't suck, they gravitate)

The material may come from, for example another star, or in the case of the giant black holes at the centre of many galaxies, from the gas, dust and stars that are found in the cores of galaxies.

As objects orbit the black hole they will tend to collide with each other, releasing energy in the form of heat, and causing them to fall to lower orbits. This process tends to cause the gas orbiting a black hole to form into a disk, called an accretion disc. Pretty soon any larger objects will be broken apart, and the accretion disk will be composed of gas, and as it heats up, plasma.

Now as objects fall to lower orbits they speed up. And for a black hole, this speed up is extreme. The gas will be orbiting at speeds that approach the speed of light. This makes friction and collisions between the particles that are orbiting the black extreme as well. The accretion disk heats up, to immense temperatures.

Now something weird happens, and the physics of it is not really sorted out. Magnetic fields get tangled up in the hot matter and cause a portion of it to be ejected away from the black hole in a jet, perpendicular to the accretion disk. The speeds of the particles in the jet is close to the speed of light. Massive amounts of electromagnetic energy is also released along this jet.

Quasars are active galaxies that happen have their jets pointing towards us. The large amounts of gas need to supply a massive black hole with the energy to make a quasar were more common in the early universe, so many quasars are very distant and very old, but the youngest is only about 700 million light years distant, and there is every reason to suppose that quasars still exist today.

You don't need a binary black hole to make a quasar, but the merging of two black holes could also release massive energy, and may be a type of gamma ray burst, and should also release gravitation waves.