A midrash (Shemot Rabbah 14:3) notices that the Torah says twice that the darkness lasted three days (Shemot 10:22-23). It concludes that there were actually two periods of darkness that were each three days long. Based on other details in the verses, it explains:
For the first three days, it was a darkness that made it impossible for people to see.
For the second three days, the darkness was so overwhelming that it wasn’t even possible to move. People were stuck in the same position for the entire three days.
