What is the ruling regarding past prayers if the worshipper performed them in a garment he believed to be clean, but later discovered there was blood on it, knowing that he had searched for blood before the prayer and did not clearly find any?

If the blood is slight, it does not affect the validity of the prayer. The amount of slight blood is estimated to be the size of a Dirham Baghalī, or that which is not excessive, or that which is considered slight by custom. If one prays while soiled with an impurity that is not excused, and he is unaware of it, he is not required to repeat the prayer.