Exodus 10:22 New English Translation

²²So Moses extended his hand toward heaven, and there was absolute darkness^[a] throughout the land of Egypt for three days.^[b]

Exodus 10:22 tn The construction is a variation of the superlative genitive: a substantive in the construct state is connected to a noun with the same meaning (see GKC 431 §133.i).
Exodus 10:22 sn S. R. Driver says, “The darkness was no doubt occasioned really by a sand-storm, produced by the hot electrical wind…which blows in intermittently…” (Exodus, 82, 83). This is another application of the antisupernatural approach to these texts. The text, however, is probably describing something that was not a seasonal wind, or Pharaoh would not have been intimidated. If it coincided with that season, then what is described here is so different and so powerful that the Egyptians would have known the difference easily. Pharaoh here would have had to have been impressed that this was something very abnormal, and that his god was powerless. Besides, there was light in all the dwellings of the Israelites.

New English Translation (NET)