If all Egypt’s water had been turned to blood, what water did the magicians turn into blood?

The passage itself answers the question. The plague struck the water on the surface, “upon their streams, upon their rivers, and upon their ponds, and upon all their pools of water”, and water standing in vessels (Exodus 7:19). It seems not to have reached water under the ground. Two verses later we read that “all the Egyptians digged round about the river for water to drink” (7:24), which implies that they found it. So clean water could still be had in Egypt, and the magicians needed only a little to copy the sign before Pharaoh. Adam Clarke answers the question in this way, from verse 24.

Keil and Delitzsch suggest instead that the magicians used water drawn before the river was struck. They note that Exodus uses ‘all’ in the same broad way elsewhere: the hail struck “every herb of the field” (9:25), yet the locusts later ate what the hail had left (10:5). Either way, the narrative does not mean that not one cup of unchanged water was left anywhere in Egypt.

The story also has a quiet irony. The magicians could add to the blood, but they could not take it away. They did not turn the river back into water, and Egypt went on digging for water for seven days (7:24-25).

Water from Goshen

An old Jewish paraphrase, the Targum of Jonathan, has the magicians take water from Goshen, where the Israelites lived. Gill thought Goshen too far away, and suggested water from an Israelite house in the city, water the magicians dug for on the spot (7:24), or a pool the plague had not yet reached, if it spread from the river outwards.

A trick

Gill and Clarke both thought the magicians may only have made a little water look, smell and taste like blood, by sleight of hand or with the help of evil spirits. On that view too they needed only a small amount of water.