Sometimes when parts of your dream are really clear and vivid you'll get these black spots or dull spots. I believe I experienced them before. I think it's just that you are concentrating a lot on what you are looking at and your mind just doesn't want to work too much on periperhal vision (I'm guessing they are off to the side?).

As for the waking up, I think I heard one person say that spots like these expanded and caused them to wake up, almost like burning away the dream layer.

Anyways welcome to DVs and yes, lucid dreaming does feel incredible.