Why does it say "Yom Choshech"?
Radak #1: The affliction is compared to darkness, just like Simchah is compared to light.
Radak #2: Amidst the multitude of locusts, the land darkened, like it says about Arbeh in Egypt "va'Techshach ha'Aretz" (Shemos 10:15).
Why does it add "Yom Anan va'Arafel"?
Malbim: The day will be dark due to lack of light. Additionally, there will be tangible darkness via vapors and smoke.
What is the comparison to morning spread on the mountains?
Rashi: The locusts spread on the mountains like morning spreads on the entire world.
Radak: Morning spreads on the mountains in a moment. The sun is seen there first, due to their height. So the locusts will spread on the land in a moment.
Malbim: It is a spark that shines to a far distance.
Why does it say "Am Rav v'Atzum"?
Malbim: It is great in quantity and awesome in quality.
It says about the locusts in Egypt "v'Acharav Lo Yihyeh Ken" (Shemos 10:14). Here it says "Kamohu Lo Nihyah Min ha'Olam" (the locusts in the days of Yo'el were worse)!
Rashi: It never occurred before that all these species came one after the other. Regarding Arbeh alone, there was never anything as bad as in Egypt.
In Egypt, the locusts ate everything within a week. Here, each species took a year to devour the crops, and [at least the first three] left a remnant. It was less intense. However, it was worse regarding the duration; the affliction lasted four years. (PF)
Otzar Midrashim (Sheloshah v'Arba'ah p.41): The Arbeh in Egypt had iron teeth, horns like an ox... (There will never be like this again. There were never natural locusts as bad as those in the days of Yo'el. - PF)
