It is an hyperbolical expression, such being very frequent both in Scripture and in all authors. For darkness being only a privation, cannot be properly felt, yet it might be felt in its cause, to wit, those thick and gross vapours which filled and infected the air. But the place may be rendered thus, that there may be darkness after that (so the Hebrew vau is sometimes used, as Micah 7:13) the darkness (i.e. the darkness of the night, or the common and daily darkness) is departed or removed, and the time of the day come; for so the root from whence this word may be derived signifies, Exodus 13:22. And to this purpose the words are rendered by the Chaldee and Syriac, and some others; and the sense is, that the darkness may continue in the day-time as well as in the night.
Related Commentaries of Exodus 10:21
Exodus 10:21
21 And the LORD said unto Moses, Stretch out thine hand toward heaven, that there may be darkness over the land of Egypt, even darkness which may be felt.