The first plague had fallen, but Egypt's astrologers refused to concede. Targum Pseudo-Jonathan on Exodus 7:22 gives a detail most translations flatten: So also did the astrologers of Mizraim by their burnings, and turned the waters of Goshen into blood.
The ordinary reading says the magicians turned some water to blood. The Targum tells us where. They went to Goshen, the Jewish quarter, the one place in Egypt whose water was still clean, and colored it red. The plague of the God of Israel had spared Goshen. The magicians' counterfeit plague reached the Israelites.
It is a bitter, brilliant touch. The Egyptian magicians could not repeal the plague or draw clean water for their own dying nation; all they could do was spoil the water of the oppressed. The meturgeman is saying something about tyranny in its death throes. When a regime cannot fix its own crisis, its last power is to punish the people it was already punishing.
And Pharaoh's heart is strengthened. The Targum uses the word taqif, strong — not simply hardened but fortified. His obstinacy is now an act of will, and it will cost him nine more plagues to see that the God who spared Goshen is stronger than any spell that could briefly stain it.
The takeaway: the powerful often cope with defeat by tormenting the weak. God sees that too, and the count of plagues keeps rising.