Blood turns a deep, dark red that is sometimes blackish in appearance when it becomes deoxygenated. If you are looking at blood not circulating inside the body, it slowly becomes deoxygenated and turns darker and darker red, explains Daven Hiskey for TodayIFoundOut.com.
Hemoglobin holds onto oxygen to carry it around your body. Hemoglobin takes on a different shape when oxygenated and appears more "red" when fully oxygenated. When the oxygen molecules leave the hemoglobin, the substance changes shape and appears to be very dark, notes Hiskey. When blood leaves your body, your red blood cells die and the hemoglobin eventually becomes deoxygenated. Blood that has recently left a body has a much brighter red color due to the fact it is oxygenated by the air. The eventual death of the red blood cells releases the oxygen, turning the hemoglobin dark red.
Your blood veins appear blue because of your skin, not your blood. Blood never takes on a bluish hue, even in its different stages of oxygenation, according to Hiskey. You see your veins as blue because of how your skin covers your blood vein and makes it appear much lighter than it actually is; the layers of your skin "lighten" the dark red blood to the point of appearing blue.