What color do you think blood is in your body? Some may find this a stupid question, but answers do vary. I find it odd that people actually thing that it's blue inside of you, so which do you think it is, honestly?
When a person donates blood, it comes from a vein, not an artery. It is also not exposed to air, or it would be contaminated and need to be disposed of. It also happens to be dark red, NOT blue.
The common misconception of "blue" blood is continued due to textbook illustrators use of blue and red to differentiate arteries and veins.
The answer is really quite complicated. There is less red light reflected from the shallow area between the surface and the vessel than blue light because blue light is scattered more. The surrounding tissue reflects back more red.
This does not make the vessel blue, however. It only makes it look blue. The authors had to use Land's retinex model of color vision to show that the contrast between the two areas of the skin are interpreted by the brain in such a way as to make it look blue.
See what I mean? People are so divided on this question. Those who say blue, always say "uhm, your veins are blue, duh" (not to pick at you Abby, you just happened to say that before I typed this). The blue believers also say unoxygenated blood is blue, but how do you find that out? You'd have to cut yourself in a vacuum, and blood carries oxygen, so how can it be unoxygenated?