Exodus 12 : 23 ; When the lord goes through the land to strike down the Egyptians, he will see the blood on the top and sides of the door-frame and will pass over that doorway, and he will not permit the destroyer to enter your houses and strike you down.
In my spiritual view, ash is symbolic of blood on body as Temple of God as per New Testament a departure from Old Testament where blood was integral in divinity oracles. It’s worthy to note Jesus’ last supper with his disciples was during Passover festivals with no meat in it.