Blue fingernails can be a sign of various disorders and should be checked out. Very cold temperatures can temporarily slow the flow of blood through the skin leading to the bluish color, but this typically goes away when you warm up. In Raynaud's disease the fingers and toes blanch, then turn blue and may become numb or painful on exposure to cold. Here, the problem is simply oversensitivity of nerves controlling blood flow through small arteries in peripheral areas of the body (fingers, toes, nose and earlobes).
Blue fingernails - the medical term is cyanosis - can also be due to low hemoglobin, the carrier of oxygen in red blood cells. Normally, arterial blood is bright red, thanks to the oxygen it contains. Your skin color, including the skin under your nails, is a combination of your skin pigmentation plus the color of your blood. When oxygen levels decline, blood turns blue-red. Chronic cyanosis can be a sign of many different lung and breathing problems, including asthma, emphysema, chronic obstructive pulmonary disease, chronic bronchitis and other respiratory disorders. It also can be a sign of a number of heart problems, as well as a response to high altitude and overdoses of certain drugs (narcotics, benzodiazepines, and some sedatives).
If your nails remain blue, I suggest having a medical checkup to identify the underlying cause. Your physician will probably want to do some tests to check your heart, lungs and hemoglobin.
Andrew Weil, M.D.