It's blue, as per this explanation:The spiders blood is different from the men's blood. In humans the oxygen is bound to hemoglobin, a molecule that contains iron and beeing red gives that colour to the blood. In spiders, and in many other arthropods (as crustaceans) and also in most mollusks the oxigen is bound to a different molecule called hemocyanin that contains copper instead of iron. The hemocyanins are proteins colourless in the reduced or deoxygenated state but the oxydized copper beeing blue/green, gives that colour to the oxygenated blood. Therefore...spiders have "blue" blood...(Dr. Mabel Rodrigues)