Isn't that an old wive's tale, and has been proven wrong so many times you'd have a greater frequency of seeing it on the 'net than a lolcat?
Ah, here we go:
Answers.com wrote:The oxidation state of the iron in the hemoglobin determines it's color; when the blood hits air the hemoglobin becomes oxygenated, so the iron's oxidation state changes, changing the color of the blood. However, the color only changes from dark red to light red, it is never blue.
It's not really blue. The changes in blood coloration you're describing relate to the respiratory pigment, hemoglobin. Hemoglobin is a topologically complex molecule that very readily binds to oxygen. Once the molecule is fully saturated, it reflects in the crimson red spectrum, and therefore fully oxygenated blood appears to be bright red.
After the hemoglobin molecule gives up its oxygen to tissue that needs it, the molecule becomes much less reflective, and thus deoxygenated blood appears very dark red to purple, or almost black. Blue is used in charts and diagrams to contrast against red, red being the arterial system bearing O2 and blue being the deoxygenated blod or venous system.
Human blood is red. Can have different hues of red, but still red.