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Retained menses: These can be another reason for black period blood. It would mean that the menses are being retained in the uterus or in vagina. It can be retained in vagina, the hymen covers the vagina completely and does not allow blood to flow out until pushed by contractions from uterus.
also one can be assured that the problem of black blood during periods is as common as the periods getting delayed or blood clots during periods so instead of getting alarmed at the slightest changes it is judicious to observe the changes over a period of time and then consult a good doctor if the need is felt