Blood appears dark when it is deoxygenated, meaning it has less oxygen bound to the hemoglobin in the red blood cells. Oxygenated blood, on the other hand, is brighter red because of the way hemoglobin interacts with oxygen. The shape of the red blood cell changes when oxygen binds to hemoglobin, influencing the way light reflects off it. As a result, deoxygenated blood, which has less oxygen bound to hemoglobin, appears darker.
The color of blood can also be affected by other factors, such as inflammation, and can vary depending on where it is drawn from the body. For instance, blood drawn during a period might appear darker than blood drawn from a vein in the arm.
