Benj: With regard to:

"Suppose a uniformly colored expanse is before me; eyes open, lights on. I am preoccupied with thoughts of math, hence am not 'attending to' the expanse or any part of it. No differentiation in judgement of any part of it from any other part of it or of it from anything ... (expand) else. No part of the expanse looks different from any other part of it; it doesn't look different from anything else (because whether I see the expanse or not, I certainly see nothing else). So I don't 'visually differentiate' anything from anything. So I don't see anything. Evidently, I have gone blind. Scary!"

I am not sure why you can't see without seeing anything.  That's what happens in a Ganzfeld, I think.   (Also, though I think you can easily reformulate the example:  I was not talking about attending or about judgement, but about seeing.)