I don't know the prayer or expression, and I don't think I have missed anything; I would guess most Catholics have never just heard of it. Catholics have a rich tradition of prayers, including the Chaplet of Mercy, given to us by Jesus through St. Faustina; it literally offers back to the Father the Body, Blood, Soul and Divinity of our Savior Jesus Christ. It is pleading the blood many times over.
Protestants have some good devotions, but nothing is better than the Rosary and the Chaplet of Divine Mercy plus, we have two thousand years of prayers and devotions from Saints, Mystics, Popes, Converts, and even the Blessed Mother.
We know and understand the power of Christ's Blood, we consume it, literally, every time we go to Mass. Protestants don't have that. That is one of the great tragedies of the Protestant revolt; they lost their heritage. If only they could turn that desire to be cleansed in Christ's Blood for the Holy Eucharist as Christ gave it to His Church, through and with a valid Apostolic priesthood, then their prayer would be answered.