A black eye is a quite common result of injury to the face or the head, caused when blood and other fluids collect in the space around the eye; swelling and dark discoloration result-hence, the name "black eye."
Most black eyes are relatively minor injuries. Many heal on their own in a few days; however, sometimes they signify a more serious injury. 
Like a bruise, as a black eye heals, the swelling around the eye decreases, and the bruise gradually fades away.

More about Black Eye in an Article HERE.
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