Hi Jarsden – interesting Q! This happens i believe because you get damaged blood vessels just under the skin, so the red blood cells leak out and spread between your skin cells. Through the skin, the dark blood looks blus-ish (in the same way that veins on the inside of your forearm look blue-ish).
I think this is because the skin (which is translucent) allows some wavelengths through more easily than others, so although the bruise is due to bleeding beneath the skin, the blood has a bluish appearence. Over time bruises also seem to change colour – it looks as if different components of the blood are spreading away from the site of the injury at different rates (like chromatography), but I don’t know if this is the real explanation.
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