The blood flowing from the capillaries into the veins on its way back to the heart and lungs have less oxygen, so blood in veins appears bluish-red. When looking at the skin, we can’t see the red arteries because they are located deep to keep them out of danger or harm. Instead, we see only veins. And they appear blue rather than red, especially in individuals with little melanin pigment in their skin (The pigment that gives human skin, hair, and eyes their colour. Dark-skinned people have more melanin in their skin than light-skinned people have).
In some people with lesser lung function, or mountaineers climbing high altitudes without bottled oxygen, the blood flowing from their heart to the tissues might be more bluish in tint than normal. This difference can be detected by looking at the colour of certain tissues with capillaries that are dense and close to the surface, such as the lips and nail-beds of the fingers. As a result, these individuals will appear to have blue lips and fingernails and this is not a good sign, because they lack oxygen in their tissues.