Although human blood is always a shade of red (except in rare cases of hemoglobin-related disease), the optical properties of skin distort the dark red color of deoxygenated blood to make it appear bluish.
The elementary principle behind cyanosis is that deoxygenated hemoglobin is more prone to the optical bluish discoloration, and also produces vasoconstriction that makes it more evident. The scattering of color that produces the blue hue of veins and cyanosis is similar to the process that makes the sky appear blue: some colors are refracted and absorbed more than others. During cyanosis, tissues are uncharacteristically low on oxygen, and therefore tissues that would normally be filled with bright oxygenated blood are instead filled with darker, deoxygenated blood. Darker blood is much more prone to the blue-shifting optical effects, and thus oxygen deficiency - hypoxia - leads to blue discoloration of the lips and other mucous membranes.
The name is derived from the color cyan, the Greek word for blue.
Acute cyanosis can be a result of asphyxiation or choking, and is one of the surest signs that respiration is being blocked.
Peripheral cyanosis is the blue tint in fingers or extremities, due to inadequate circulation. The blood reaching the extremities is not oxygen rich and when viewed through the skin a combination of factors can lead to the appearance of a blue color. All factors contributing to central cyanosis can also cause peripheral symptoms to appear, however peripheral cyanosis can be observed without there being heart or lung failures. Small blood vessels may be restricted and can be treated by increasing the normal oxygenation level of the blood.