From this page:
Unlike human blood, which is iron-based, Vulcan blood is copper-based, making it green. Red blood cells in humans store oxygen by means of hemoglobin, which makes them bright red; that is why human arterial blood, on its way from the cardiopulmonary system, is brighter red than venous blood, which has carried its oxygen throughout the body's tissues and is on its return trip for re-oxygenating. Copper molecules, when oxidized are green--that's why copper pipes turn green eventually. Hence, green Vulcan blood. Vulcan venous blood, on its way back to the heart, is more copper- or rust-colored, containing less oxygen. Vulcan blood cells are also shaped differently, more like lentils, whereas human blood cells are concave.
If Spock is half human half Vulcan, why is his blood makeup that of a full Vulcan?
Did they perform gene therapy prior to his conception or are Vulcans and humans compatible by default with the Vulcan traits being dominant?