The first plague was blood. The Sages explain why God began specifically with this plague, and describe how conspicuous the difference was between the Egyptian and Israelite camps while the plague ran its course.

Why were the Egyptians afflicted with the plague of blood first? It is because Pharaoh and the Egyptians worshipped the Nile as a god. The Holy One, blessed be He, said: I will smite their god first, and the people thereafter. The Israelites became wealthy from the plague of blood. An Egyptian and an Israelite would be in one house with a vat filled with water. The Egyptian would go to fill a jug from the vat. He would remove the jug filled with blood. The Israelite would drink water from the vat. The Egyptian would say to him: Give me a little water with your hand. The Israelite would give it to him, and it would become blood. The Egyptian would say to him: Let you and I drink from the same bowl. The Israelite would drink water and the Egyptian, blood. When the Egyptian would purchase it from the Israelite with money, he would drink water; the Israelites became wealthy from selling water to the Egyptians. (Shemot Rabba 9)