The plague of “darkness” defied nature. Normally, we would define darkness, as the absence of light. But in Egypt, the darkness was actually felt.
The Torah describes that not only were the Egyptians unable to see, but they were also unable to move. What added to the miracle, was that the Jewish people had light.
It was at this time, that the Jewish people were able to see exactly what was inside each Egyptian home. This made it easy for them to see what items they would “borrow” when they left to the Promised Land.
The Chidushei Harim has a different lesson to be learned from the plague of Darkness. The worse darkness is when one doesn’t wish to see his brother’s pain, and reach out to help. The result of such apathy is that when one ignores such hardships of others, he becomes stuck himself.
He fulfills the other half of the verse, in that he becomes stuck and unable to move. He is trapped in his own darkness, and is in a hopeless situation.
It is very sad that there is so much darkness in the world, which includes so much selfishness and lack of consideration for others. We must see the “light” and learn how to see others and act as the Torah demands.