.

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.

"...the bride of Christ, follows Her Lord in His life, death, and resurrection.  The Church goes through Her passion and death (seemingly),  three days of darkness as in a tomb,  and Her resurrection being the triumph of the Sacred Heart of Jesus and Immaculate Heart of Mary. ...

I know this isn't the best Thread to post, but the matter is related. I just happened to find a French  website: Mouvement Catholique des Familles, and I see a couple of good assessments in its Homepage. 
https://www.m-c-familles.fr/90-mcf/199-mgr-lefebvre-fidele-et-rebelle

AI translation. 
Excerpts:


Bishop Lefebvre as an example 
Introduction 
   Citing in an issue of 'Famille d'abord' (their Newsletter Td. ) dealing with authority, will seem to some superficial minds an incongruity, even a provocation. How can the bishop who opposed the authority of Paul VI by refusing the new mass, and even more so how can the one who stood up against the authority of Vatican II in a work entitled "I accuse the Council" (Ed. Saint-Gabriel, 1976), give the example of obedience?

   Superficially or in the media (these are synonyms), we can see in Bishop Lefebvre a rebel, a proud, and be scandalized by it. Superficially or dialectically (these are also synonyms), we can see in him an iron bishop resistant to all the assaults of modernism, and congratulate ourselves.  The Liberals will try to reconcile these contrary points of view by saying that Bishop Lefebvre was "faithful in his rebellion" or "rebel in his fidelity". Let's leave them this strangely reversible language, like their convictions...

Deeply, Bishop Lefebvre was faithful to what he had received and had to transmit, as he had it engraved on his grave: "tradidi quod et accepi" (1Cor 15,3). He opposed what contradicted what he had received and that he had to transmit faithfully. He was rebellious when he was forbidden to be faithful. Does he free himself from authority in the name of a free personal examination, according to subjective criteria? No, as he wrote in his statement of November 21, 1974: "No authority, even the highest in the hierarchy, can force us to abandon or diminish our Catholic faith clearly expressed and professed by the magisterium of the Church for nineteen centuries. "If it happens," said St. Paul, "that we...."

...His disobedience to the Roman authorities was dictated by fidelity to the authority of God. And it was always a source of suffering for him, unlike the revolutionaries who are happy to see the authority itself destroyed,  ....

For the touchstone of this disobedience out of fidelity is the cross. The cross not placed on the wall, but "planted in the heart of existence", as written by P. Calm. It is she and she alone who shows the attitude to have in the crisis that the Church is experiencing. During a sermon delivered for the ordinations in Ecône, in 1982, Bishop Lefebvre recalls that on Calvary Mary was standing at the foot of the cross where was nailed the One who no longer had human appearance and whose divinity seemed to have disappeared, but who remained true God and true man. Similarly, when the Church goes through her passion, when her divine authority seems to disappear under the actions of overly human authorities, she is always the mystical Body of Christ, and only the eyes of faith allows us to stand.

There are Catholics who suffer for the Church and do not want to suffer by the Church. There are others who suffer for the Church and end up forgetting that it is for the Church. Bishop Lefebvre suffered by the ecclesiastical authorities for the authority of the Church.

The founder of the Fraternity of St. Pius X always refused to be considered the leader of the traditionalists, out of humility, that is to say out of fidelity to the eternal authority of God, to the bimillennial authority of the Church that he wanted to serve without deviating an iota. In this sense, according to the expression of P. de Broglie, he did not lead, but we went towards him to keep the course he faithfully held. He was a model; he remains an example.