No, human blood is never blue. Even when it is totally lacking in oxygen, it is a dark red color. With oxygen, it is a bright red. OK, then why do your veins look blue? First, your veins are not transparent, so you cannot see the red blood through them. Your veins are not blue either, but scattering of light and partial absorption of the red end of the spectrum causes them to look blue when seen through your skin. Some creatures, such as horseshoe crabs, have blue blood, but mammalian blood is red all the time.