What can cause your blood to be a dark rust color?

Methemoglobinemia: a condition in which an abnormal amount of methemoglobin is present in the blood, producing a dark or chocolate colored blood. Causes may include exposure to certain chemicals (e.g. nitrites), certain medications (e.g. dapsone, antimalarials, sulfonamides), or congenital methemoglobinemia (rare genetic condition).