Prayer: Mixing Another Impurity with Excusable Blood
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 846
If the area of blood on one’s body or clothing is less than the area covered by a dirham and another impurity reaches it – for example, a drop of urine falls on it – then in case it reaches a pure part of the body or clothing, it is not permitted to perform prayers with it. In fact, even if it does not reach a pure part of the body or clothing, it is not correct to perform prayers with it based on obligatory precaution.