There are several possible reasons for someone to experience blood in the urine (called hematuria). Urinary tract infections can cause inflammation of either the bladder or the urethra which can lead to irritation and bleeding
from the mucosal surface of these structures. Kidney stones
are another possibility. These stones are usually made up of calcium, and can either cause local trauma within the kidney or in the ureters (the tubes connecting the kidneys to the bladder). Kidney stones are usually associated with some form of pain when they are large enough to cause hematuria. Less commonly, inflammation within the kidney due to either an infectious or non-infectious process (such as autoimmune kidney disease) can cause red blood cells to leak into the urine.
Something else to consider is that dark urine may not contain blood at all. Extremely concentrated urine due to dehydration can sometimes appear dark and be confused with blood. Elevated levels of bilirubin (which can occur with liver disease or the excessive breakdown of red blood cells) can also cause the urine to appear dark.
It would be advisable for you to seek out the help of your primary care physician
to help determine the cause of your hematuria and come up with an appropriate treatment plan.