15 Go to Pharaoh in the morning, as he is going out to the water. Stand on the bank of the Nile to meet him, and take in your hand the staff that turned into a serpent. 16 And you shall say to him, ‘The Lord, the God of the Hebrews, sent me to you, saying, “Let my people go, that they may serve me in the wilderness.” But so far, you have not obeyed. 17 Thus says the Lord, “By this you shall know that I am the Lord: behold, with the staff that is in my hand I will strike the water that is in the Nile, and it shall turn into blood. 18 The fish in the Nile shall die, and the Nile will stink, and the Egyptians will grow weary of drinking water from the Nile.”’” 19 And the Lord said to Moses, “Say to Aaron, ‘Take your staff and stretch out your hand over the waters of Egypt, over their rivers, their canals, and their ponds, and all their pools of water, so that they may become blood, and there shall be blood throughout all the land of Egypt, even in vessels of wood and in vessels of stone.’”20 Moses and Aaron did as the Lord commanded. In the sight of Pharaoh and in the sight of his servants he lifted up the staff and struck the water in the Nile, and all the water in the Nile turned into blood. 21 And the fish in the Nile died, and the Nile stank, so that the Egyptians could not drink water from the Nile. There was blood throughout all the land of Egypt. 22 But the magicians of Egypt did the same by their secret arts. So Pharaoh's heart remained hardened, and he would not listen to them, as the Lord had said. 23 Pharaoh turned and went into his house, and he did not take even this to heart. 24 And all the Egyptians dug along the Nile for water to drink, for they could not drink the water of the Nile 25 Seven full days passed after the Lord had struck the Nile (Exod 7:15-25).
A couple of preliminary points before I get to the main point:
i) Hebrew has the same word for blood and the color red. Therefore, it's prejudicial to say the Nile transmogrified into hemoglobin.
ii) Some well-meaning people attempt to defend the historicity of the plagues by construing them naturalistically. But although some miracles employ natural mechanisms, some of the plagues are designedly discriminating in a way that defies a naturalistic explanation. The plague of blood is case in point. Consider v19. The implication is that the plague extended to water that was collected prior to the plague. There's no natural process by which water in separate containers could become contaminated after the fact. That's independent of what happened to the Nile.
iii) V24 is intriguing. Unbelievers think Exodus is pious fiction. Even if they think it contains a kernel of historical truth, they believe it's mostly legendary embellishment. And the miracles are, from their perspective, paradigm examples of legendary embellishment.
But why would a narrator writing pious fiction invent v24? Doesn't that circumvent the miracle? Even if it was understandable for Egyptians, in their desperation, to dig down to groundwater to find potable water, we wouldn't expect the narrator to let them succeed. Rather, if even water in containers was contaminated, we'd expect the groundwater to be contaminated. Why would the narrator invent that loophole?
This is the kind of niggling detail that only makes sense if the account is factual. God allowed Egyptians to find drinkable water because it wasn't his intention to make all the Egyptians die of thirst. Rather, the point of the plague was to send a message: to show that Yahweh was the true God, a God with awesome control over natural forces and natural elements. A God who could best the Egyptian pantheon on their own turf.
Perhaps the groundwater was naturally filtered. so that it escaped the effects of the plague. God didn't make the plague extend to groundwater. The miracle didn't impede the normal filtration process that purifies polluted surface water from potable groundwater. But that's a realistic detail you wouldn't expect if the account is pious fiction.