I remember growing up hearing the words for a song....signs, signs, everywhere there are signs.
The blood of the covenant relationship is a sign, a symbol, a signal of God's presence in this relationship. "I AM the Lord your God, who led you out of slavery."
Then Moses took the blood from the basins and splattered it over the people, declaring, “Look, this blood confirms the covenant the Lord has made with you in giving you these instructions.”
Later, Jesus affirms the new covenant through his own blood. Paul recites for us as we do every time we share the covenant meal of Holy Communion; "and after supper he (Jesus) took the cup, blessed it, and gave thanks, and gave it for all to drink saying this is the new covenant in my blood shed for you and fo all people for the forgiveness of sins....do this in remembrance of me.
The Sacrament always acts as a sign of God's presence, real presence in this blood, confirming for us the life giving act through the blood to confirm for us our desire to see time and time again God's activity in the life of the world.