There are a few different reasons that your nails may turn blue. Cyanosis can happen for a variety of reasons, such as heart or lung disease. Another possibility is when the weather is cold, your blood vessels constrict in order to keep your body warm.  This can cause the veins in your hands and feet to become more visible, which may make your nails look bluer than usual. A third reason why you may have blue nails is because of a blood cell abnormality.