The distinction is a bit like the Mary's Room thought experiment.

In case one (never having had any sight), the person has no way of "knowing" what black is. What they see is nothingness.

However, a person who knows what black is (that is, having seen it before), and who therafter loses sight, might describe the nothingness as "black," as either pure white or pure black is the general "color" that we associate with nothing.

A large part of this distinction comes from the fact that the visual cortex of any given person who has never had sight would be underdeveloped due to lack of reinforcing signals coming from the optic nerve - the "use it or lose it" function of brain mapping, especially from an early age. Note that this is in addition to never having actually experienced what "blackness" is. The person who loses sight later in life would still have much of the visual processing neurons intact (unless damage to the occipital lobe was the cause of the vision loss) - and would retain all the knowledge of color (or lack thereof) that a sighted person would.

Another way of looking at the distinction: In the absence of light, we experience blackness. Imagine if you had the ability to, say, sense electric fields. You would probably have a level of sensation that would correspond to "no field present." If you never had this ability, you would not be sensing "no field," you would simply sense nothing at all. However, if you had the ability and then lost it at a sensory level, your bran (which must have set up some relays in order to process the signal in the first place) would interpret that lack of sensory data as "no field" rather than nothingness.