פירוש על שמות 7:22

בלטיהם SECRET ARTS — magic formulas which they utter secretly and in a whisper. Our Rabbis stated that בלטיהם refers to the work of demons, בלהטיהם to witchcraft (Sanhedrin 67b; Exodus Rabbah 9:11).

ויעשו כן חרטומי מצרים, The Egyptian sorcerers did likewise, etc. If we follow the sages in Shemot Rabbah 9,11 according to whom all the waters in Egypt were struck, where did the Egyptian sorcerers practice their art and convert water into blood? Perhaps only visible sources of water had been turned into blood, whereas subterranean water had remained in its original state. This would explain why the Torah described the Egyptians as digging for water in verse 24.

ויעשו כן חרטומי מצרים, “the sorcerers of Egypt did likewise.” From where did these sorcerers take the water to duplicate Aaron’s turning the water into blood? They must have either used water that the Egyptians had dug up surrounding the areas that contained water, or they must have taken water in possession of the Israelites. Some commentators claim that the waters that Aaron had turned into blood reverted to being water in short order, so that there was no shortage of water anywhere. This would account for the fact that Pharaoh failed to be impressed with this miracle. This is why Moses and Aaron were not called in to reverse the plague, (as they were on the next occasion).