The medical term for the bluish hue of the skin, including that underneath the fingernails, is cyanosis. This can be the result of hypoxemia a low amount of oxygen-carrying hemoglobin within red blood cells. Normally, arterial blood is flush with these oxygenated cells, making your blood appear bright red, but when oxygen levels decline, blood contains less red blood cells and turns blue-red. Because your skin is a combination of your skin pigmentation and the color of your blood, cyanosis is an indicator of this deficiency and is often first noticed on the lips, gums, around the eyes, and under the nails.
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. Blue fingernails or purple nail beds can be a sign of various disorders and should be checked out. In the case of Raynaud’s disease, the fingers and toes blanch, then turn blue and may become numb or painful upon 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).
Chronic cyanosis also can be a sign of many different lung and breathing problems, including, a pulmonary embolism, asthma, emphysema, chronic obstructive pulmonary disease (COPD), or chronic bronchitis. Heart problems, exposure to high altitudes, and reactions to certain drugs, such as minocycline used to treat bacterial infections , narcotics, benzodiazepines, and some sedatives. Be aware that taking colloidal silver can also cause the appearance of a slate-blue color just above the lunula (half-moon in the nail bed), an irreversible skin condition called argyria.
If blue nails aren’t merely a temporary reaction to cold temperatures, I suggest having a medical checkup to identify the underlying cause. Your physician will probably want to do a simple rapid-blood gas analysis to measure the oxygen in your blood and perhaps some tests to check your heart and lungs.
Andrew Weil, M.D.
http://emedicine.medscape.com/article/303533-overview http://www.mayoclinic.org/symptoms/hypoxemia/basics/causes/sym-20050930 http://www.mayoclinic.org/symptoms/hypoxemia/basics/definition/sym-20050930 http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3033929/ http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3582910/ http://www.niams.nih.gov/Health_Info/Raynauds_Phenomenon/ https://www.nlm.nih.gov/medlineplus/druginfo/meds/a682101.html https://www.nlm.nih.gov/medlineplus/ency/article/003215.htm