Those who are having some knowledge of biology might say veins are blue because the blood in veins is deoxygenated. While it's true that the blood in veins carry a lower concentration of oxygen than the arteries, this isn't the reason for their blue appearance in your skin.
Another argument that you might hear is that – ‘we do not see blue blood because the moment blood comes out of veins, it get oxygenated after getting in contact with the air and turns red.’ This again is a false explanation for their coloration. Blood is never blue.
A surgeon will tell you that while doing surgery, veins within the body do not appear blue either. So, not only is blood not blue, veins on their own aren't blue either. That means "Blue colour veins" is a phenomenon that is visible from outside the skin.
So, what's going on here?
The colour is basically the phenomenon of how light interacts with our skin and blood, what portion of that light gets absorbed and what gets reflected.
Skin does not absorb much light at any wavelength. Blood, on the other hand, absorbs light of all wavelengths (but less in the red part of the spectrum, hence it appears red).
In case of blood vessels, they are around .5mm deep below the skin and at that depth, blue color of the light gets absorbed less and gets reflected more, giving the veins a blue coloration.
None of this, however, addresses the question of why only veins appear blue and not the arteries. The reason is that veins are the only vessels we actually observe through the skin. Arteries on the other hand are much deeper.
The point here is that the bluish appearance of veins has everything to do with their location below the skin and accordingly what light they reflect and absorb. It has nothing to do with the concentration of oxygen within them. In fact, if we could see them through the skin as well, even arteries would look blue.
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