That's why children instinctively know that when they spot a monster under their bed or in their wardrobe, all they have to do is to turn the lights on. This will instantly blind the black-eyed monster, who will have to run to his ophtalmologist in order to cure his wounds. Hence my question : are ophtalmologists more prone to see black-eyed people ?
Seriously. I think Charliebrown is right to point out the importance of physics, even when we consider a "supernatural" phenomenon. We tend to call "supernatural" things we don't understand, but it doesn't mean that these phenoma, should they be proven real, are above the laws of nature.
It is therefore sensible to admit that, if fully black-eyed people do exist, this would have some impacts on what stands as their "body". The blackness of these eyes would have a purpose. If it doesn't, it may simply mean that this purpose is of symbolic nature. And if this purpose is symbolic, as we don't live on symbols, it is all the more likely that such or such creature is rather a cultural creation, or a myth.
So on the physical plane, what could explain pitch black eyes ? Sunglasses, for instance. A black-eyed being perhaps hides is true eyes under a sunglass-like device we mistake for his eyes. Or perhaps, such a creature would have black eyes because he lives in a very luminous environment. By comparison, the Earth being extremely dark, he has to widen his pupils to the extreme in order to adapt his vision. Or perhaps, as it has been shown on various experiments that humans tend to find people with enlarged pupils more attractive, black-eyed people would widen their pupils in order to influence us (and awkwardly achieve the opposite ...).
On the symbolic plane, there is a common saying that the eyes are the "windows of the soul". So pitch black eyes would be indicative of a dark soul or worse, as absence of soul. We find plenty of red-eyed creatures in the Western folklore for the same kind of reasons.
I suspect black-eyed kids are just a urban legend, and therefore, their black eyes are simply a symbol of their lack of soul.