what causes blood to turn black
Too much iron will usually turn the blood black. However, if there is liver failure, and it's seeping into the blood stream that too can cause the blood to turn black as it becomes over loaded with toxins. Put simply it's the blood is overloaded with toxins and other nutrients that the body can't process entirely.
There is also a chance that your black period blood is caused by an infection or STD. Uterus infections can cause blackening as well as many STDs. Women that have sex during their cycle also experience black bleeding because the blood was pushed back into the vagina and can become trapped as a result.