A black butterfly is a butterfly often symbolizing death. Though it does not mean "to die" but to have something come to it's end. These are said to appear to foretell the end of something, and said to appear to one who is near death. They are both mythical and real. I mean mythical because of the stories accompanied with them, and they are real because there are many butterflies who are many black.

when you're hungover as shit after a spending a night drunk as a skunk

Pussy that got fucked so much it looks like a floppy old black butterfly.