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You are absolutely correct that it doesn't. And the passage refers to the end of the world, not the three days of darkness.

The three days of darkness is not in Scripture.

Incorrect, the Tribulation is not the end of the world.  The Tribulation is in the 'end days' but there is a period of peace, after God intervenes (which is what this passage is referring to, God's intervention against His enemies, after they persecute Christianity).  The anti-christ comes AFTER the Tribulation.

All of the prophecies of the '3 days' speak of God's wrath towards his enemies.  God gives them their day and then His justice steps in and cleans house.  There there is peace.

There are many things which "aren't in Scripture" which are still catholic.  The number of prophecies about God's wrath coming with 3 days of darkness, with a comet, with earthquakes/floods, etc (all of which are implied in the above biblical passage) are ALL TIED TO THE PERIOD OF TRIBULATION.  

The whole purpose of prophecy is to 'fill in the gaps' of Scripture, for the elect who are living in those times. 
Prophecy = hope. 
Prophecy = God's mercy. 
Prophecy = God's Providence in action, telling us, "Don't worry, I have things under control.  I have told my saints to prophecy these events CENTURIES before.  Nothing will happen unless I allow it."

The '3 Days of Darkness' is a hopeful message.  The only people who should fear are God's enemies.  Catholics will win because of the '3 Days'.  Christianity will return because of it.  As Christ said, (paraphrasing) "As in the days of Noah, so will it be when the Son of Man returns." 

The flood was God's way of wiping the slate clean of evil and starting over.  The '3 Days' will be the same, but more than just 8 people are going to survive.

Not sure if it is applicable in a metaphorical sense to the Three Days of Darkness, but I am reminded of the horribly agonizing three days of "darkness" Mary and Joseph suffered through as they searched for the Child Jesus.  Luke 2:45-6