"...The darkness has blinded his eyes."

A couple month's ago I was reading my Bible and I came across this verse... 
"But whoever hates his brother is in the darkness and walks in the darkness, and does not know where he is going, because the darkness has blinded his eyes." ~1 John 2:11
I've always wondered what that meant and I thought it meant that they couldn't see spiritual things until we went to TN "Lost Sea Adventures". "Lost Sea Adventures" is a cave where you can learn about caves and go on a boat ride around a lake in it. While we were their our guide turned off the lights and told us that if you stayed in that much darkness for two weeks you would go blind. This verse means that walking in that kind darkness so long that they went blind.