to the op
I learned that the expression 'I plead the blood' came from the blood of the lamb on the doorposts of the Israelites when the angel of death passed over before the exodus from Egypt
I don't think it's actually a biblical expression but I learned the above while attending a Pentecostal church for awhile (I'm not Pentecostal but I had friends there)
I do think the following is based on scripture and explains better why the blood of Christ is effective both for our salvation and protection. (Derek Prince)
we should not walk around afraid of the devil. he is already defeated but from time to time we might encounter spiritual warfare and it because of Jesus shed blood that the enemy has no hold over us any longer
Through the blood of Jesus, I am redeemed out of the hand of the devil. Through the blood of Jesus, all my sins are forgiven. The blood of Jesus Christ, God’s Son, cleanses me continually from all sin. Through the blood of Jesus, I am justified, made righteous, just-as-if-I’d never sinned. Through the blood of Jesus, I am sanctified, made holy, set apart to God. My body is a temple for the Holy Spirit, redeemed, cleansed, sanctified by the blood of Jesus. Therefore, Satan has no place in me, no power over me, through the blood of Jesus. I renounce him, loose myself from him and command him to leave me in the name of Jesus. Amen.