Black-and-blue describes the dark purple, blue, and sometimes greenish colors that appear on your skin after you've been bruised. When you bang your shin hard against a table leg or take a tumble off your bike, blood vessels under your skin break and leak blood into the surrounding tissue. That trapped blood shows through your skin as a bruise, creating those telltale black-and-blue marks.

The colors change as your body heals. A fresh bruise might look reddish or purple, then turn darker blue or black, and eventually fade to green, yellow, or brown before disappearing completely. This rainbow of colors happens because your body is breaking down and reabsorbing the leaked blood.

People use the phrase to describe being covered in bruises: “After hockey practice, my legs were black-and-blue.” Sometimes the phrase emphasizes how badly someone was hurt in a fight or accident: “He was hurt black-and-blue.” The term captures that distinctive look of a serious bruise, where the injury creates such dark colors that they really do look almost black against the blue-purple background.