Why can we see the color black?

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Because black isn’t black. What?

Things appear dark because they reflect less light than other things, but our brains have a pretty generous definition of what “black” is. Supposedly, the darkest color should reflect no light at all, but since we never encounter anything that reflects no light at all, the concept of “black” is pretty vague.

Everything reflects some light. Take a look at this black car, for instance:

That’s seriously black…or is it? Notice how you can see the curves and shape of the car by the way the light reflects off of it. It doesn’t reflect as much light as a white car, but you can still see it. It actually looks more greyish than black, because it’s outdoors where there’s a lot of light to reflect. It would look much closer to black in a dark room, but so would everything else.

All right, let’s find something blacker - specifically, a substance called vantablack, which reflects almost no light at all:

These are two nearly identical objects, but one of them is coated with vantablack, which absorbs 99.96% of all visible light. That’s about as close to pure black as you can get - if that were in empty space, it would be pretty hard to see. And yet, if you look at it just right, you can still make out the contours of the face. That 0.04% of visible light is still enough to see.

Now look out at the empty space beyond the Earth’s atmosphere:

That’s a real photo taken from the International Space Station. You’d think empty space would be the darkest thing you could find, since it’s pretty much nothingness. And yet, if you look at it, you’ll see the light of billions of stars across the entire universe. There is not a single spot you can look at in any direction where you’re not seeing some source of light, even if it’s billions of light-years away.

So, to answer your question, we can see “black” because what we think is black isn’t really black. There is no matter that we know of that reflects zero light (except maybe dark matter, which we’ve never been able to find because, well, we can’t see the dang stuff). Thus, a human eye has never seen a truly black object, and most likely never will.