A ''black eye'' is a bruise to the eyelid skin caused by blunt trauma to the eye region. Like many bruises, a "shiner" may be nothing to worry about and might disappear on its own in a few days.
In some cases, however, a black eye is a warning sign of more serious injury to the eye or to the skull. Any damage to the eyeball that causes it to become red and swollen must be promptly evaluated by a doctor or an eye specialist. Blunt force eye injuries, as happens in fighting, competitive sports, and ordinary accidents, could involve an unsuspected detached retina, internal bleeding, or other serious problems. A fracture involving the delicate bones around the eye may trap an eye muscle or soft tissues. An orbital fracture could damage the optic nerve and permanently damage eyesight. If so, emergency surgery may be required to correct the condition.
Most black eyes are the result of blunt trauma that causes bleeding beneath the thin eyelid skin, producing the characteristic black and blue discoloration. A fracture deep inside the skull can also blacken both eyes, even though the eye area itself was not injured. People with sinusitis from allergies sometimes get "allergic shiners" -- darkening under the eyes caused by inflamed and engorged blood vessels.