Is my prayer valid, given the continued discharge of menstrual blood for 10 to 14 days, and its turning black in the first days, and should I abstain from marital relations?

A woman's usual period may increase or decrease. As long as the blood continues from ten to fourteen days, all these days are considered menstruation, during which she abstains from prayer and fasting, and it is not permissible for her husband to approach her. The increase is considered menstruation from the first instance.