So can you, for example, picture a black square which takes up the entirety of your vision? So your eyes are open, but you perceive nothing in front of you?
Not quite like that, but yes, sure.
I can decide not to process what comes in my eye, as long as it's not a tiger or something. Then I don't see it. If it's a familiar face or a threat, then it's harder to phase out, almost impossible.
It's useful to have something better to fidget with than a black square. But I can do black square.
Also your actual vision is like a spot, not like a square.
This is how I went from not being aware "picture yourself on a beach" was more than a figure of speech to being able to decompose mechanisms and take measurements all in my mind.
This is exactly how you do it.
Your eyes have nothing to do with it. Your brain fakes your eyes seeing stuff for you, and it can fake other things for you if you ask it.