Or HaChaim on Exodus 10:22/אור החיים על שמות י׳:כ״ב

1ויט משה את ידו, "Moses inclined his hand, etc.," We need to understand why Moses had used his staff to bring about the plague of locusts (verse 13) when G'd had told him to incline only his hand (verse 12). There had not been a single plague so far which was introduced by Moses being told to use "his hand" rather than his staff.

2Perhaps the reason why this plague was different is best explained in Shemot Rabbah 14, according to which the darkness in Egypt belonged to the category of darkness described in Psalms 18,12: "He made darkness His screen, dark thunderheads, dense clouds of the sky were His pavilion around Him." Inasmuch as the darkness was of a supernatural kind, Moses did not consider it appropriate to raise his staff against supernatural phenomena.

3Other rabbis, commenting in the same section of that Midrash, considered this darkness as belonging to purgatory. Both groups of rabbis may have been correct; there were two categories of darkness. One prevailed during the three days no Egyptian could move, the other during the days they merely could not see one another.