You see an area that looks dark.
It might be hard to describe, because it doesn't exist in the real world, but I honestly don't know why it seems so hard to imagine. It's dark. It's a dark area. It doesn't get brighter when you hold a torch up to it, but you can see light sources on the other side. What does it look like? It looks like whatever is there, only everything is a whole lot darker, so details are hard as heck to make out. You might not be able to see anything in the area at all, or maybe you can. That all depends on what's going on, where it is, and what's behind it. All we know is that it's dark, and it looks dark.