How long does it take frostbite to turn black?

1-3 days: Initially you get white, red, and yellow patches that become numb. Blisters develop 1–2 days later, then become hard and blackened (but usually appear worse than they are). If the area freezes further, deep frostbite occurs, causing damage to muscles, tendons, blood vessels, and nerves. That is a serious situation, sometimes resulting in loss of fingers or toes.

2-3 days after: The initial thermal cold injury is when tissues can be black. Still even seemingly damaged tissues may survive. Tissue survival is a main difference between a medium deep frostbite and a severe frostbite. But both can be black after a few days. This is why most doctors will tell you to let the tissue "demarcate" before doing anything. The tissue is watched as healing takes place.