Why does frostbite make skin turn black?

Lack of blood: The skin turns black because there is a lack of blood flow to the area with frostbite. Frostbite causes blood vessels to vasoconstrict (close). With no blood flow to the affected area, the skin becomes necrotic, which causes the skin to turn black.

It's dead tissue: Dead tissue is black. The end result of severe frostbite is dead tissue.

Black Skin: Extreme cold is directly toxic to the skin cells and the blood nourishing the cells. The skin dies and dead skin is black.