The other possible explanation of this groping or feeling is that this darkness was from a very, very bad sandstorm. Khamsin, Winds of the Desert, are not unusual in Egypt. These can come up suddenly and may last for up to three days. But Khamsin do not cover the entire country and they never block out all of the light. With the khamsin, sand is blown into everything, including homes.
People have to stay in the inner parts of the house to get away from the sand.
If God did use a khamsin, it was the worst that the world has ever known. It could be felt and it completely blocked out any light from the sun, moon and stars. Whatever caused the blackness, Pharaoh knew it was from the One True God. God had demonstrated that he was more powerful than the sun god, Ra. Ra was the father of the gods.