1) This may seem obvious, but the color of your 'superficial veins' (the ones you can see) is clearly due to the blood that is in them. You can prove this to yourself by finding an obvious vein, say on the back of your hand, and stripping the blood out of it by pressing on it and pushing toward your finger tips. The vein disappears, but will reappear as soon as you lift the pressure and the blood is allowed to re-enter. Cool, huh?

2) It is further clear that the oxygen content of the blood in the vein is a big player, since whole parts of your body can turn blue that are normally pink (at least in a caucasian). Doctors call this cyanosis. For example, kids with heart defects can turn their whole body blue, and people with certain "rheumatologic" disease can have periodic episodes where their normally pink fingertips turn blue. In general, these things happen because the amount of oxygen in the blood in the capillary beds drops below the normal level. By inference, then, the blue color of veins that you see is due to the low amount of oxygen *normally* in that blood.

3) HOWEVER, oxygen poor blood is most definitely NOT blue. As you point out, it is a sort of purplish/maroon color at best. I work in the clinical labs, and have seen countless tubes of blood drawn from oxygen poor veins into vacuum tubes (i.e. never exposed to air), and it has never looked like the color of your veins!

4) Your thought experiment about the appearance of veins in surgery is a very good one. Basically, no, veins do not appear blue in surgery. But understand that the types of veins you see through your skin are never really dissected in surgery, they're too small and thin. Nonetheless, many veins do get dissected, and they typically have a reddish appearance. But paradoxically, they often keep that appearance even after the blood is washed from them, since they are inevitably coated with amounts of loose connective tissue called fascia which has blood in it and is thus red. So in the end this question is confounded, but again, they never appear truly blue.

5) So we seem to be narrowing it down. Skin must be having an effect. Can we find more evidence for this? I think that varicose veins are a help here. We have all seen people with 'spider looking' dilated veins in their legs. While their appearance is a certainly variable, not infrequently they are sort of a maroon color, much like the blood appears. These veins are dilated and have stretched their walls and the overlying skin, and as a result are very close to the surface. And they are no longer blue!!

So, what all these little bits of evidence say is that both the oxygen content of the blood AND the overlying skin affect the apparent color of the veins you see. Realize that to see the vein at all, light has to go THROUGH the skin and hit the blood in the vein. The blood absorbs certain colors of light, and reflects others back through the skin. In the skin, it is potentially subject to all sorts of effects, including scatter by the 'stratified' layers, and absorption by the pigmented layers (i.e. melanin). For some reason, the combination of these effects (absorption by oxygen poor blood and absorption/deflection in the skin) gives a blue color. I'm not sure if anyone could say in absolutely precise terms why blue, but there it is.

A long-winded answer, but hopefully enough to satisfy even a true 'blue-blood.'

Thanks for the question.