As the ten plagues progress, they get increasingly severe. The initial plagues (blood and frogs) are nuisances, but not outright threats. However, the later plagues, such as those of cattle, locusts, and the slaying of the firstborn, are particularly severe. Why, then, is the ninth plague, the second-to-last and thus second-most severe, three days of darkness? (Ex. 10:21-27)
I say this: imagine trying to do any of your normal tasks in complete darkness, without even starlight or candlelight to guide you. Imagine trying to work the field with sharp objects, slaughter animals, or even care for children. It's theoretically possible, but the darkness must have been so intense that many Egyptians probably never moved from their homes for the full three days. The effects of such paralysis must have been truly severe.