The Black Chests appear in Paper Mario: The Thousand-Year Door. They are black-colored treasure chests that need a black key to open. Each contains a being that pretends to be helpful to get Mario to free them, but really plans to curse him. The beings are yellow faces with dots for eyes and a squiggly mouth on a navy blue background with black dots. The curses all help Mario, though, so the beings aren't as evil as they think. All of the curses utilize Mario's paper form, causing him to turn into such things as a paper airplane or sailboat.
The 4 beings inside 4 Black Chests are actually the 4 heroes that defeated the Shadow Queen 1,000 years ago. They became Black Chests when the Shadow Queen used the last bit of magic she had to curse four of the Crystal Stars and when one of the four heroes hid one of the Crystal Stars, they turned into Black Chests. The fact that they curse Mario and seem to have become "evil" in general may be a result of anger created by the suffering they endured in the chests.