This is actually not an easy question to answer, and it depends. Remember that sight is in the brain, not in the eye. For example, hallucinations mean that you see something that isn't actually there, but you're still seeing that thing...it just has no real analog in the external world. Sight can't really be separated from things like attention and memory, or even the input from other senses.

One woman I know formerly had limited vision and now has no vision, even light perception. However, subjectively she sees colors that she says are very beautiful. When she holds a card from her wallet, she'll say it looks green. Does it actually look green? Yes, because green is in the brain. When electromagnetic radiation reflects off that card, is it within the wavelengths on the human-visible spectrum that we've decided to call green? No. But her brain has hooked up some equipment that she formerly used to perceive radiation in the environment to some of her other senses.

Daniel Kish, who uses active echolocation for navigation, rightly describes what he perceives through sound as sight. That's because all evidence points to his using the same neural equipment that sighted people use for sight. Since sight is in the brain, he sees things through echolocation.

So your question sort of assumes that sight is like a movie or a show, a series of images that, like JPEGs or GIFs, have some objective numeric value. But, really, human experience is the product of a cascading series of links forged and links cut off in the brain. That means that what blind people see is not really black, and it's also not really nothing. The answer is that seeing is really just an abstraction of what's really happening that's useful when discussing most issues day-to-day but which breaks down when you try to talk about something like what blind people see.
 

So here's two short answers to your question:
 

Q: What do blind people see?
A: Not electromagnetic radiation.
 

Q: What do blind people see?
A: The world.

Hi,

It’s kind of a tough question, but I appreciate you feeling free to ask it; I will try to answer it from my own experience. First of all, even though I was born with sight, I believe your description of the experience of someone who was born blind is, largely, correct. My wife, for example, was born blind and she does not have personal experience with sight. However, I do not believe this is as big of a deal as you might think. For one thing, I would not say that she has no concept of vision, no more than I would say that a sighted person has no concept of an atom. Of course, it’s impossible to “see” an atom, but we all have a concept of an atom thanks to the alternative ways we have experienced atoms. The same thing is true for my wife and color. If she says the word black, the color, itself, does not come to her mind, but she does know the concept through reading, living in society, and being taught from a young age what black means. So, for her, she may think of formal classy clothing, or something you would wear to a funeral, or nighttime, or mystery, or the opposite of light etc. In fact, even though I had sight until I was 18, my wife might be better at saying what colors go with what, because she had to make a point to learn such things, where as I took that kind of thing for granted as a sighted teenager.

My wife was one of the first people I ever got to know well who was born blind, and I’m ashamed to say that when we first became friends I thought there would be obvious examples that illustrated that she had never experienced sight. However, for the most part, that just is not true.

Now for me, on the other hand, what I experience as a totally blind person who was sighted until I was 18 is different. For me, I’m never really experiencing blackness either, as if I was a sighted person with my eyes closed; there are a number of reasons for this. For one thing, most of the time I see blobs of glittery colors shimmering around. Make no mistake, these glimmering shapes are not some tiny bit of vision that I still have, these colors are just my broken eye doing odd things, perhaps, a bit like the static that you would get on a broken old-school TV. You could wave a flash light, or the sun, in front of my face, or turn all of the lights out, and it would have no effect on the glittery things I’m “seeing”. Such optical phantoms, sometimes called floaters, are pretty common for people who lose their sight. It’s funny, when I lost the vision I had, due to a surgery, these glittery lights were so bright that I had trouble sleeping for a few nights. I remember being annoyed thinking, I thought being blind would consist of a lot of blackness, but here I am being kept awake by phantom lights. This feeling of being annoyed only lasted for a few days, because my brain quickly learned to make the lights go away when I need them to. Most of the time, as I go through my day, I’m not staring at glittery lights, they are often still there, but if I don’t think about them or pay attention to them they might as well not exist; by the way, since I’m thinking about them right now they are pretty bright, silver, some dark blue, purple etc. Most of the time, when I’m just going through my day, I am seeing a vague visual idea of what I’m doing. When I am making coffee, I have this idea of the coffee pot in front of me, my cup in my hands, and my cat at my feet. Now, even though I’m experiencing things in an imaginary visual way, I’m not conches that I’m doing so, and I would not have super specific details to give most of the time, unless I purposely thought about them. In other words, I have this picture in my head of me sitting on my couch typing this, but if you asked me what color my shirt was, I would have to think about it, even though I see my shirt in my mind’s eye. I think when I first lost my sight my imagined pictures were more specific, but now my brain gives me the basic visual layout, unless I ask for more.

One last thing I will say is, when I think of my memories as a sighted person, and my memories as a blind person, they are not very different. You might think that there would be some great drop or change in the quality of your experiences after you lost your sight, but for me that is not really the case. I will say that I think I am a pretty visual person, and that being a blind person makes it a little harder for me to remember things, but that is more of a practical matter, instead of a major difference in my experience.

So, in conclusion, no I’m never really just experiencing blackness, either because I’m seeing pretend lights, I’m seeing a vague pretend picture of what I’m doing, or I’m thinking about a concept or memory and such thoughts are very visual for me. I want to make it clear, everything I have said just comes from my own experience, and my understanding of my wife’s experience. Everyone’s experience is going to be different, so I cannot speak for anyone else.

What blind people see is largely dependent on how their brains learned to receive and process information. I never had sight in my right eye, so on that side it is like trying to see with my elbow, on the other hand, I lost the sight in my left eye when I was 25 and as such the brain still wants to receive information and as such I have similar visualizations to the commenter who saw blue and silver, though mine are purple and whiteish gray swirling over washed out black, sometimes with a bit of green.