Is the profuse, odorless black blood that is discharged four days after intercourse, following a period of purity, considered menstruation or a discharge from a vein?

The summary of the ruling regarding recurring blood is that if its total, along with the previous bleeding and any interspersed days of purity, exceeds fifteen days, then it is Istihada (prolonged non-menstrual bleeding). If this total is fifteen days or less, then it is Hayd (menstruation). If it exceeds fifteen days, then you are a Mustahada (a woman experiencing Istihada). You should do what a Mustahada does: refer to your previous regular period if you had one. Otherwise, you should act according to sound discrimination. Otherwise, you should consider six or seven days as menstruation, which is the most common duration for women, and whatever exceeds that is Istihada. You should perform Ghusl (ritual bath) after the calculated duration of Hayd ends, then perform Wudu (ablution) for each prayer after its time enters, taking precautions. You are subject to all the rulings of pure women during the period considered Istihada, and the purity interspersed between two blood flows is a valid purity.