What causes your blood to be black in color? Just had blood drawn and it was black. Person drawing it has never seen blood this color.

Methemoglobinemia. This condition can be inherited or acquired. Exposure to triggers like benzene, antibiotics like dapsone or chloroquine, nitrities used as a preservative in meats or anesthetics like benzocaine and xylocaine (lidocaine) can cause it in acquired cases. Symptoms include bluish tint to skin, shortness of breath, fatigue, lack of energy, and headaches. Vitamin C, hyperbaric oxygen, and methylene blue to treat.
Methemoglobin. This is one possible explanation for dark blood; classically methemoglobinemia looks like chocolate. If it actually looks black, perhaps it was the dark-dark-red blood of an athlete's veins seen in light with more blue wavelengths. If you feel okay, I wouldn't be concerned.
It wasn't black. Truly black blood is impossible, but blood that has high oxygen has a more reddish color and blood that has less appears darker; it can almost appear black. Blood can appear very dark brown with methemoglobinemia, which you might have if you are taking too much Tylenol (acetaminophen) or had exposure to certain "caine" drugs for example for endoscopy. Stop taking tylenol (acetaminophen). See ED if short of breath or ill.