What had happened was that the city was struck by tornados just before the seder began second night, and the storm created power outages throughout the area. I was worried about the kids. At home Shifrah still gets very agitated by power outages and is afraid of the darkness that comes on so suddenly. But at Passover, once the sirens stopped, the kids actually enjoyed conducting seder this way. Surrounded by their grandparents, aunts, uncles and cousins, conducting the seder by candlelight was actually very cozy.
Darkness can be very frightening, particularly when it is unexpected.
For that reason, it worked as one of the plagues. It forced Pharaoh to consider giving in to the Israelites request, that they be allowed to travel three days into the wilderness to worship their God. The darkness that struck Egypt was a very thick darkness, different from the darkness of a normal night. It is described in the Bible as something one could touch, vayamesh choshech. Those of a more naturalistic bent identify the plague of darkness with the desert chamsin, the hot sorocco winds that often blow fine sand so thickly that it is impossible to see even one's hand in front of one's face.