This condition is known as cyanosis.
It occurs when there isn’t enough oxygen in your blood, making the skin or membrane below the skin turn a purplish-blue color.
The skin discoloration could also mean that there’s a high level of an abnormal form of hemoglobin in your blood.
Hemoglobin is the protein responsible for transporting oxygen in your blood.
Cold temperatures can also cause your fingernails to turn blue.
The narrow pathways make it difficult for enough oxygen-rich blood to get to your nails.