Albert Barnes Commentary Exodus 10:21

"And Jehovah said unto Moses, Stretch out thy hand toward heaven, that there may be darkness over the land of Egypt, even darkness which may be felt." — Exodus 10:21 (ASV)

Darkness - This infliction was specially calculated to affect the spirits of the Egyptians, whose chief object of worship was the Sun-god. Its suddenness and severity, in connection with the act of Moses, mark it as a preternatural withdrawal of light.

Yet, it has an analogy in physical phenomena. After the vernal equinox, the southwest wind from the desert blows for some 50 days. It does not blow continuously, however, but at intervals, generally lasting some two or three days each time. This wind fills the atmosphere with dense masses of fine sand, bringing on a darkness far deeper than that of our worst fogs in winter.

The consternation of Pharaoh proves that, familiar as he may have been with the phenomenon, no previous occurrence had prepared him for its intensity and duration. He recognized it as a supernatural visitation.