Sforno on Exodus 10:21/ספורנו על שמות י׳:כ״א

1נטה ידך על השמים, “in the direction of the atmosphere referred to as “heaven.” We have explained this in connection with Genesis 1,7.

2וימש חשך, this will remove the normal darkness called “night.” The reason this was necessary was that the night consists of air ready and capable to absorb light in the morning. The darkness that would occur now was something unable to interact with light at all. The reason for this inability to interact with light was the density of the texture of this darkness. As a result of this totally different kind of darkness לא ראו איש את אחיו, for even a lit up flare would not be able to make a “dent” in that darkness.