The common misconception that blood that lacks oxygen turns blue probably comes from the fact that veins appear blue and blood in the veins is typically heading back to the lungs, hence depleted of oxygen. It is also possibly from the fact that when people are suffocating or oxygen deprived, their lips and complexion seem to turn blue-ish or purple.
People who perpetuate this myth often claim that the reason we never see blood in its blue form is that the instant we get cut, the blood is exposed to oxygen and thus, instantly turns red. Of course, when you get blood drawn from your veins that isn’t exposed to air, we see that it is dark red, but let’s not let such observations get in the way of science.
Blood is Never Blue
In reality, it turns out that when blood is deprived of oxygen, it actually just turns dark red, as we often see when it is drawn when we are donating blood. When it’s oxygenated, it turns a brighter red. This red color primarily comes from the hemoglobin, which contains four heme groups. These heme group’s interactions with various molecules end up giving it the dark red or light red color we see. The hemoglobin itself is a protein that binds with oxygen to eventually be distributed throughout the body in blood.
Veins are very close to the surface of skin, which helps them play a role in heat dissipation (or vasodilation). Due to this closeness to the surface of the skin, they are very visible through the skin. This location in the skin is largely why veins often appear blue despite the fact that the blood running through them is dark red. Specifically, this is from the way light diffuses in the skin. Veins appear blue from the way subcutaneous fat absorbs low-frequency light. This permits only high frequency blue and violet wavelengths to penetrate through the skin to the vein, with the other wavelengths getting filtered off from the pigmentation of your skin. In fact, when skin is removed, such as when dissecting a cadaver, the veins and arteries actually are a somewhat similar color.
Bonus Science Stuff
While people with, for lack of a better word, “medium” skin color will typically have blue veins, veins often won’t appear blue if a person has darker or lighter skin. Rather, the veins tend to appear green or brown. On the other hand, people with extremely light skin, such as an albino, will typically have veins that show up as dark purple or dark red, more closely resembling the actual color of the blood running through the veins. Again, this difference is due to the way that light diffuses through different skin complexions.
A similar light diffusion process is happening when we see people who are suffocating and have purple lips and fingertips. In this case, the blood is so oxygen deprived that it turns an extremely dark shade of red. When the light diffuses through the skin of the lips or through your fingernails, it then often ends up looking dark blue or purple.
Blood from a person suffering from carbon monoxide poisoning is bright red because the carbon monoxide results in the formation of carboxyhemoglobin, which results in the blood turning bright red and being unable to carry oxygen from the lungs, due to the fact that it will be unable to release the carbon monoxide. The hemoglobin itself binds to carbon monoxide much better (240:1) than oxygen, which is part of the reason it is so dangerous.
[Reposted (with editing) from Misconception Junction]