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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

I'm reading in the book of Exodus 10:22
And Moses stretched forth his hand towards heaven: and there came horrible darkness in all the land of Egypt for three days.
23. No man saw his brother, nor moved himself out of the place where he was: but wheresoever the children of Israel dwelt there was light.

This could be a foreshadow of the defeat of the antichrist, Pharoah being a type of antichrist and all the woes and plagues of the last days similar to Moses bringing plagues on Egypt.  When after the tribulations of those days, the sun shall be darkened and the moon shall not give forth light."
Three days of darkness where only blessed candles in the homes of the children of God will have light.

Yes, as I said in an earlier post. The Three Days of Darkness relates also to the period of the Two Witnesses in the Apocalypse. See the similarities below:

Apocalypse 11:6 These have power to shut heaven, that it rain not in the days of their prophecy: and they have power over waters to turn them into blood, and to strike the earth with all plagues as often as they will.

Exodus 9:33 And when Moses was gone from Pharao out of the city, he stretched forth his hands to the Lord: and the thunders and the hail ceased, neither did there drop any more rain upon the earth.

Exodus 7:20 And Moses and Aaron did as the Lord had commanded: and lifting up the rod he struck the water of the river before Pharao and his servants: and it was turned into blood.

Exodus 8:16 And the Lord said to Moses: Say to Aaron, Stretch forth thy rod, and strike the dust of the earth: and may there be sciniphs in all the land of Egypt

This means that the Three Days of Darkness comes AFTER the Two Witnesses, which is mentioned in this verse:

Apocalypse 11:9 And they of the tribes, and peoples, and tongues, and nations, shall see their bodies for three days and a half: and they shall not suffer their bodies to be laid in sepulchres.