What cause finger nails and toenails to turn blue?

Raynaud's: Raynaud's disease can cause this. It is a temporary vasoconstriction of the small arteries in the fingers and toes triggered by cold.

Vasoconstriction: Vasoconstriction of the vessels can cause nails to turn blue. Known as raynaud's phenomenon, the constriction causes a loss of bloodflow in the area and improper oxygenation. The bluish color is described sometimes cyanotic. Another cause of blue nails can be from trauma due to a subungual hematoma (blood blister under the nail).