Dark blood is deoxygenated blood, which means it arises because there is a lack of oxygen, according to the University of California, Santa Barbara. Blood is always red, but the shades vary according to the stages of the circulatory system.
All blood cells contain small amounts of iron, which turns red when it comes into contact with oxygen. This is why blood is usually bright red, as blood cells pick up oxygen when they pass through the pulmonary circulation and they carry it to cells around the body.
While blood in the arteries usually contains a lot of oxygen, blood in the veins is bringing carbon dioxide back from the body's tissues so it can pass through the lungs and heart to acquire oxygen again. As a result, it is usually darker than the blood in the arteries. The same applies to blood in the capillaries that are feeding the veins.
Once deoxygenated blood reaches the heart again, it moves through the right atrium and into the right ventricle. From there, the right ventricle pumps it into the pulmonary circulation, where it leaves the carbon dioxide behind and picks up oxygen. Oxygenated blood then enters the left atrium and the left ventricle, before being pumped into the circulation again to deliver oxygen around the body.