blood: God could not simply spare his people from this plague as he had with the others. Death rules in the world because of sin. In light of God's justice, sin cannot be ignored. He must punish or atone for it.
Blood represents life (Leviticus 17:11). So, it is the only acceptable way to forgive sins (Hebrews 9:22). As Deuteronomy 6:9 suggests, doorframes represent the entirety of life as the place of entering and exiting.