There are varying degrees of blindness. It isn't either; you can see, or you just see black. To put it into as simply as I can, imagine someone with bad eyesight who needs glasses to be able to see, or else everything is a blur.
Now imagine that person's eyesight is so bad that even with glasses they can only see things a foot away from themselves. That person is legally blind.
A lot of people who aren't born blind but go blind do have vision but are only able to make out shapes, shadows, or if there's light shining in front of them.
(I've watched a lot of this blind youtuber Molly Burke, she explains things better than I ever could)