An image whose thumbnail in Danbooru is simply a black rectangle.
It happens when a transparent background becomes black in the process of resizing the image, but the rest of the image is black as well, and/or is merely an outline too thin to be viewed as a thumbnail.
The real contents can be viewed when the image is in its ORIGINAL size. (i.e., if a message like "This image is displayed at 28% of its original size." above the image, then you should click "View original")
Example: In post #1076703, a catgirl is hidden, but viewable when the image is in its original size.