Is the black blood that comes after purification from menstruation to be considered istihada (irregular vaginal bleeding), making prayer and fasting obligatory, knowing that it is distinguishable from istihada and that the minimum period of purity has not passed?

Blood connected to menstrual blood, whether it be a brownish discharge (kudra), a yellowish discharge (sufra), or black blood, is considered menses as long as its total duration does not exceed 15 days. So, if the blood is seven days, the yellowish and brownish discharge three days, and the black blood five days, all of this is considered menses because it did not exceed 15 days. As for blood seen 11 days after the cessation of menses, it is irregular bleeding (istihada), and it cannot be combined with what preceded it to form a single menstrual period, nor can it be considered a separate menstrual period, because the minimum period of purity (tuhur) between two menstrual cycles is 13 days according to the Hanbalis and 15 days according to the majority of scholars. During the days of irregular bleeding, one must perform ablution for each prayer after its time has begun, and pray with this ablution while taking precautions (tahaffuz), and you have all the rulings applicable to those in a state of purity.