Last night, I was observing a portion of the Pipe Nebula, which is a dark lane of dust obscuring the Milky Way. It was kind of cool: I could see stars to the left of it through the telescope, and then as I moved the telescope to the right, the field of view went almost completely dark, except for some stars on the fringes and some quite faint stars in the middle. But did I see the Pipe Nebula? It seems that what constituted "seeing" the Pipe Nebula was my not seeing the stars behind it. After all, intuitively seeing seems to be a causal process whereby the seen object causes light to reach the eyes. But the Pipe Nebula did not cause any light to reach the eyes. Of course, the same issue comes up when one "sees" a matte-black cube, a shadow, etc.
Maybe, then, we need to relax the intuitive concept of seeing as a process whereby the seen object causes light to reach the eyes. One might say that seeing is a process whereby the seen object causes light to reach or not reach the eyes (or, maybe better, causes a particular profile of wavelengths of light to reach the eyes, which profile might be empty). But if that were right, then we should say that a blindfolded person sees the blindfold (and that a person with eyes closed sees the inside of the eyelids). However, I do not think we say that—we say, rather, that a blindfolded person does not see anything.
Here is an alternative that accounts for shadows, ultra flat black cubes and dark nebulae: in seeing, the seen object causes a non-empty shaped pattern of light. There are two ways of causing a shaped pattern of light: one way is by causing the light (by reflection, emission or refraction) and the other way is by causing the shape. Here, "shape" must be understood in such a way that a field of view filled with uniform light counts as a shape (so that if one is right up against a uniform blue wall, one still sees the wall) while an empty field of view, as in the blindfold case, does not count as a shape.
A problem with this account is that if one's face is up against a red, or green, or blue wall, one counts as seeing the wall, but if the wall is painted with ultra flat black paint, then one isn't seeing the wall. I do not know how to give an account on which one counts as seeing the ultra flat black wall, but one doesn't count as seeing the blindfold.