It seems that black pixels could be achieved using LCD technology. Even of just to augment the display.
You can easily convince yourself that this won't work for an eye. Of course, you can use your finger to block out any object - as long as you hold your finger well away from your eye. Now take a match, and try to block out objects by holding it a centimeter in front of your eye; it doesn't work - you will see a diffuse slightly darkened area, but all objects will be visible.
It seems then that the best option would be a device which affects your neural cortex more directly.