There was an apt resemblance in this plague to what Israel had long experienced from the furnace of their oppressors. So God himself calls it: Deuteronomy 4:20.
Related Commentaries of Exodus 9:8
Exodus 9:8
8 And the LORD said unto Moses and unto Aaron, Take to you handfuls of ashes of the furnace, and let Moses sprinkle it toward the heaven in the sight of Pharaoh.