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When blood has just come from the lungs after taking on oxygen it has a bright red color. As blood flows through the body, oxygen is released to the various cells in the body to keep them alive. As the blood loses oxygen, it turns a darker shade of red.

Scientists know this because they see the differences in color between the blood they sample from the vessels that carry fresh blood from the lungs to the heart and the color of the blood sampled from the vessels that carry blood back to the lungs from the rest of the body after it has given up the oxygen to the tissues.

Have you had blood drawn out of your blood vessels? The blood goes into a sealed container. If the blood is drawn from a vein the blood will be dark red / blue. That is because there is little to no oxygen in that sealed container.