Being completely blind, with no light perception whatsoever, is extremely rare. Most blind people still get some visual input ranging anywhere from only being able to perceive very bright lights, to quite a lot of vision (the 20/200 threshold of legal blindness).
Most people who have absolutely no light perception are that way because the connection between their eyes and their brain is completely cut off. This is usually because the eyes have been removed, because the optic nerve has been completely severed, or because of brain damage. These people will not see black. Seeing is what your brain does with signals that your eyes send to the brain. If there is no signal, nothing is seen. A good question to ask yourself is, "What can I see with my elbow?" You don't see black with your elbow. You don't see with it at all. There are some exceptions to this.
Sometimes people who are totally blind will "see" flashes of light or color that aren't there. It's just a biological glitch, kind of like the flashes or sparkles people with normal vision see temporarily if they get hit on the head.
Sometimes people who experience vision loss later in life (but not people who were born blind) have a type of non-psychotic hallucination called Charles Bonnet Syndrome. People who have CBS know that the odd things they see are not real, they are simply a brain glitch, and they do not affect any other senses. The hallucinations are usually of tiny versions of everyday objects or people. Because this usually happens to older people, people who have not been told by their doctors about the possibility of CBS may fear that they are developing dementia. Fortunately, CBS is not harmful or degenerative, and is simply a small annoyance that people adjust to over time.