As horrifying as a black eye can look, it is usually nothing that will cause lasting or serious damage. A black eye is technically a hemorrhage around the eye; the blood that's underneath the skin shows through as a purplish color.
There are many diseases and conditions that can cause a black eye, but a true "shiner" is typically caused by some trauma to the eye. Anyone with a black eye should see a doctor to rule out damage to the eye itself. If you have any vision problems, it is especially important to get to the doctor immediately.