Its because light has to penetrate the skin to illuminate the veins, blue and red light (being of different wavelengths) penetrate with different degrees of success. What makes it back to your eye is the blue light. Oxygen-rich blood is bright red, oxygen-poor blood is not so bright :/. Blood is always red.
You don't always see them, but you know they're always there. Life was meant for good friends and great adventures. I would rather walk with a friend in the dark, than alone in the light. A good friend is like a four-leaf clover: hard to find and lucky to have. Good night!