What is the colour of unoxidized blood?

The story behind the answer

Red. In humans, blood is always red because of hemoglobin, an iron-containing protein in red blood cells. When oxygen is bound (oxyhemoglobin), it looks bright red; when unoxygenated (deoxyhemoglobin), it’s a darker, maroon red—still red, not blue.

Veins can look blue through skin due to how light penetrates and scatters in tissue, not because the blood is blue. Other animals do have differently colored blood: copper-based hemocyanin makes some arthropods’ and mollusks’ blood blue; chlorocruorin can make some worms’ blood green; hemerythrin can appear purple. Memory tip: Iron makes human blood red—bright with oxygen, dark without—but never blue.