When you close your eyes for a very long time, or allow them to adjust to the non-light of a cave or your bedroom at night, the "blackness" you see is not actually very black, objectively speaking. If, in a well-lit room, you were to look at a large, matte-black object, you would actually end up perceiving that object to be significantly blacker than simple pitch-darkness. Human perception of colour is highly dependent on the context in which any given colour is perceived, and very dark colours are only perceived as being particularly dark, because they are adjacent to things which are not dark in colour. When you are in a very dark room and can see nothing at all around you, your brain is not able to discern a context for the blackness you perceive, and as a result, your brain settles for regarding the darkness as being uniformly a dark charcoal grey. This grey is known as Eigengrau.
Eigengrau is the "intrinsic grey," the baseline colour perception which the brain uses when deprived of all colour context. It is visual white noise, filling in the empty spaces when the brain is not receiving adequate visual stimulation to resolve the image into discrete shapes and colours. Given long enough to stare at a uniformly matte-white wall or any other colour which consumes the entire field of vision, without differentiating shadows, bright spots, or other hues, the brain can eventually turn any field of any colour into a perceived eigengrau, instead of the "true" colour of that field.
If you ask a person with acquired blindness (not necessarily somebody born with impaired or absent vision) what they "see," and they are aware of the concept of eigengrau, this would be the answer they might give you. It is the answer I would give you. On days when the apperception lets up enough for me to detect shapes and colours, the brights are always brighter and the blacks are always blacker than the flat dark grey I usually experience, or the "static and snow" I get when the nerves fire randomly.
Our primate brains are exquisitely good at processing information regarding differences and details in any structure, surface, shape, or texture. Delicate variations between two kinds of green or yellow could, in our ancient past, indicate whether a plant was safe to eat; minute differences in shadows might give away the location of a predator. The brain is always hunting for these differences and for movement of any kind, and when there is no movement to see and no difference to observe, it will sometimes resort to falsifying visual hallucinations (or auditory hallucinations, such as have been observed in anechoic chambers) to create differences and movement. It takes much longer spans of time without stimuli before the brain completely abandons these hallucinatory efforts and settles on perceiving nothing but flat empty greyness. In other words, if you are trying to see eigengrau intentionally by staring at a blank wall in a well-lit room, be prepared to potentially see some trippy shit before it happens. . . or just wait for nightfall, go to bed, close your eyes with all the lights turned off, and enjoy the welcoming quiet dark as sleep claims you.