Hemoglobinuria: The presence of free hemoglobin in the urine may make the urine look dark. Normally, there is no hemoglobin in the urine. Hemoglobinuria is a sign of a number of abnormal conditions, such as bleeding and paroxysmal nocturnal hemoglobinuria.
From 
References
Jameson, J. Larry, et al. Harrison's Principles of Internal Medicine, 20th Ed. New York: McGraw-Hill Education, 2018.