The Targum Pseudo-Jonathan on Exodus 10:23 reveals a secret buried in the ninth plague that the plain Torah only hints at.

"No man saw his brother, and none arose from his place three days. But among all the sons of Israel there was light, that the wicked among them who died might be buried, and that the righteous might be occupied with the precepts of the law in their dwellings."

Two different Egypts existed in those three days. The Egyptian Egypt — paralyzed, blind, unable to stand — and the Israelite Egypt, where every home was lit.

The Aramaic paraphrase, preserved in the Targum attributed to Yonatan ben Uzziel, adds two astonishing details. First: some of Israel's own wicked were dying during this time, and the darkness gave the righteous cover to bury them. Which wicked? The midrashic tradition tells us — the Jews who had assimilated so deeply into Egyptian life that they had refused to leave when the Exodus began. The Lord took them during these three days so that Egypt would not later say, "Look, these Jews died in our plagues too, proving their God was powerless."

Second: the righteous used the three days to study Torah. Mit'askin b'pikudei oraita — occupied with the precepts of the Law. While Egypt sat frozen in blindness, Israel studied.

The Maggid teaches: the plague of darkness was also a plague of sorting. Egypt was shown what it was. Israel was shown what it could become. And the work of becoming a Torah people began in the very shadow of Pharaoh's throne.