What is the ruling on the prayer of those who prayed with blood, believing it to be pure, if the correct view according to God is that blood is impure?

There is no sin upon one who adopts the opinion of a qualified mujtahid (independent jurist) in matters of juristic discretion (ijtihad).

Differences in rulings between the Imam and the congregant do not invalidate the prayer of either of them; each is excused for what they have acted upon.

Based on this, it is permissible for adherents of some of the four madhhabs (schools of thought) to pray behind others, despite their differences in some jurisprudential issues.

If the congregant is certain that the Imam performed an action that is not permissible according to their own understanding, the prayer is still valid. This is the view of the majority of the early Muslims (Salaf). The Prophet, peace and blessings be upon him, clarified that the Imam's mistake does not extend to the congregant: "They lead you in prayer; if they are correct, it is for you and for them, and if they err, it is for you and against them."

The congregant believes that the Imam has exercised ijtihad or followed a mujtahid, and that Allah has forgiven his mistake. Thus, the Imam's prayer is valid, and likewise, the congregant's prayer is valid.