Garbage bag
The resolution on nanny cams can sometimes leave a lot to be desired. You have to become adept at translating the grainy images into what is actually going on in the room. Occasionally a tired parent can end up confusing something innocent for a danger.
In this case, however, the danger wasn’t an alien, an oil spill, or a split to another dimension. The “huge black thing” was a garbage bag that the baby had somehow climbed out of his crib to collect. Why he thought he needed a garbage bag in his crib I’ve no idea, perhaps he felt he was getting crowded out by toys, or maybe he just had high standards of cleanliness.