Is the blood that changes color from light red at the beginning of menstruation to a darker, blackish color in the following days considered istihadah, necessitating wudu and prayer?

The default rule is that any blood a woman sees is menstruation (hayd) as long as it is suitable to be hayd. It is only deemed to be irregular bleeding (istihada) if it is impossible to classify it as hayd. Menstrual blood is a natural disposition and a recurring custom, while istihada blood is due to an incidental illness. All blood a woman sees is considered hayd if it occurs during a time suitable for hayd, meaning its duration does not exceed fifteen days, and there is a gap of fifteen days or more between its appearance and the cessation of the previous bleeding.