Hebrews 1:3, Who being in the brightness of his glory, and the express image of his person, and upholding all things by the word of his power, when he had by himself purged[ cleansed] our sins, sat down on the right hand of majesty on high.
Hebrews 9:12, Neither by the blood of goats and calves, but by his own blood he entered in ONCE into the holy place having obtained eternal redemption for us.
Verse 25, Nor yet that he should offer himself often as the high priest entered into the holy place every year with blood of others.
Verse 26, For then must he often have suffered since the foundation of the world, but NOW ONCE in the end of the world hath he appeared to put away sin by the sacrifice of himself.
In the OT the blood of goats and calves only covered the Hebrew peoples sin and rolled them back that the penalty of their sin would not come up them. They had no cleansing of sin.
So that old term , under the blood is a habit of saying and not a reality of truth.
Even though I know what they mean it should be corrected and put in proper tense. Tell the devil " The blood of Jesus has cleansed me from all sin. And the righteousness that I now have is the righteousness of Jesus.
I no longer am a sinner saved by grace, I'm a saint because he has sanctified me, justified me, and declared me righteous through his blood. I have within me a new nature, one created after him and I no longer practice sin as a habit. But in all things acknowledge him in whom I have to do.