AKA: Oil Is Usually Black, Right?
Ah, the Black Oil. One of the most key components of The X-Files' internal mythology, and yet it didn't show up until over halfway through the third season. It's definitely one of the most memorable aspects of the mythology, visually distinct and utterly unique from most of the previously established alien mythology (which, aside from the Alien Bounty Hunter, have been pretty straightforwardly pulled from general UFO ephemera). And its introduction would mark a major turning point in the story.