As messengers of God Moses and Aaron have to speak to Pharaoh: ’Let my people go, so that they may worship me in the wilderness’. Pharaoh refuses to acknowledge God and let His people go. Then a confrontation follows. God demonstrates His omnipotence to Pharaoh by causing plagues over Egypt. These plagues will eventually force Pharaoh to let the people go.
The first plague is the changing of Egypt’s waters into blood. When Pharaoh goes out to the river in the morning, Moses and Aaron must confront him on the bank of the Nile. They must make it clear to him that due to his refusal to let the people go, the water of the Nile will be changed into blood.
Before Pharaoh’s eyes, Moses (or Aaron) strikes the water with the staff that earlier had been changed into a snake. And the water, of the life source of Egypt, changes into blood, so that the fish die and the smell of death rises.
Pharaoh’s wizards cannot change the blood into water again, but seem to be able to turn water into blood as well, so Pharaoh sticks to his refusal. He hardens his heart and chooses not to acknowledge God.
-Moses acts as the ‘mediator of the Old Covenant’ who brings the punishment of God upon the oppressor. In the end he will lead the people out of Egypt.
Moses is a foreshadowing of the ‘Mediator of the New Covenant’, who, in Rev.19, is called ‘the Word of God’. He is the person riding on the white horse, who executes the judgements over the gentiles. That is why his robe is ‘dipped in blood’. (19:13)
Verse 15 says: 'He treads the winepress of the fury of the wrath of God Almighty'
In Rev.16:6-8 we read about a judgement of God where the water is changed into blood. According to 16:6 this is the retribution for the shedding of the blood of the holy people and the prophets.
The changing of water into blood in Ex. 7 may be likewise retribution for the drowning of the new-born Hebrew boys in the Nile.
In this first plague water is changed into blood. The first sign/wonder Jesus worked in public was the changing of water into wine. John 2. In the Last Supper Jesus connects wine to blood when He says: 'This cup (filled with wine) is the new covenant in my blood, which is poured out for you. Luke 22:20. His blood was shed for us when He carried the just judgement in our place.