Why is my blood dark?

Myoglobinuria: Very, dark urine with the color of cola could be due to the presence of myoglobin, which is a breakdown product of muscle cell. It can be caused by trauma, alcohol, medications, and vigorous exercises. It could indicate that you have rhabdomyolysis, which could lead to kidney failure. Another cause would be blood (as in uti) or bilirubin in the urine (as in liver disease). Should be checked.

See below: What makes you think that your blood is dark? Venous blood normally has darker color than arterial blood.