Take a bit of dust way above a black hole. It's got a lot of gravitational energy. Say that the piece of dust is starting to fall towards the black hole. It is now lower in a gravitational field, so it must have less gravitational energy. Some of the energy also went into heat.
Hot objects radiate light, from a hot stove emitting infrared to a hot poker emitting visible light.
The friction between the bit of dust and the other bits of dust around the black hole translates into more heat and more light being emitted. After a while the bit of dust starts to visibly glow. It falls some more, and soon it gets so hot that it emits x-rays. Astronomers have telescopes that scan the sky for x-ray sources and have found no few black holes that way. So, that's why it goes so fast.