Your deoxygenated blood is blue
The old claim turned out to be wrong and was rejected by evidence.
TaughtBlood carrying oxygen is red, and blood returning to the heart is blue. That's why your veins look blue, and why the diagram has red arteries and blue veins.
NowHuman blood is never blue. Deoxygenated blood is dark red, close to maroon. Veins look blue because of how skin scatters and absorbs light, not because of what is inside them.
What actually happened
The optics are the whole explanation. Skin scatters shorter wavelengths and absorbs longer ones, so at the depth of a typical vein, red light is preferentially absorbed by the tissue and by the blood itself, while blue light scatters back to your eye. The vein is a dark object seen through a scattering medium, and the result reads as blue. Journal of Vision work in the 1990s modelled this quantitatively.
The direct evidence is trivially available. Blood drawn from a vein into a tube is dark red the instant it appears, with no time to "turn red on contact with air" as the folk version claims. Anyone who has had blood taken has seen this. Venous blood is still about 75 percent oxygen-saturated, which is why it is dark red rather than pale.
The red-and-blue diagram is a legitimate teaching convention, and it works. The error was reading the convention as a description. Some animals genuinely do have blue blood: horseshoe crabs and octopuses use copper-based hemocyanin instead of iron-based hemoglobin. We do not.
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Who was taught this
Still standard through 2005, so anyone who finished school between 1950 and 1995 learned the earlier version.