What causes blood in a blood draw to appear black? when i had my labs drawn my blood was black. the lab tech even commented on the color, stated "she had never seen blood that dark before."

May be: you have some lung dz but you did not tell me if smoke.Also get checked for any heart murmur or shunt. if your ABGs are normal then may be not imp.

It may be poorly: oxygenated blood from a vein, the more deoxygenated the darker the blood looks. Also high levels of bilirubin, a breakdown product of heme in red blood cells, can cause darker looking plasma, which will make overall color darker crimson. But it should not be "black" like the color of the sky at night.