Why does blood turn black when it dries?

Blood appears dark red or black when it dries due to the oxidation of hemoglobin, the protein in red blood cells that carries oxygen. When blood is exposed to air, it loses moisture and the iron in hemoglobin undergoes a chemical change, causing the color to shift. Additionally, the presence of other components in blood, like cells and plasma, can contribute to the darker appearance as it coagulates and dries. The visual change is a natural result of the drying and oxidation processes.