I have heard of a young SSPX priest debunk the 3-Days of Darkness, because he said it was not in Holy Scriptures.  (How Protestant the thinking!)

That's an odd perspective, given that one of the plagues of Egypt was a 3 three days of darkness, which is mentioned in the Book of Exodus: 


"And the Lord said to Moses: Stretch out thy hand towards heaven: and may there be darkness upon the land of Egypt, so thick that it may be felt. And Moses stretched forth his hand towards heaven: and there came horrible darkness in all the land of Egypt for three days. 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" (Exodus 10:21-23). 


Also, this three days of darkness is described in more detail in Chapter 17 of the Book of Wisdom. I recommend reading that if you want to learn more about the Three Days of Darkness. Here are some excerpts: 


"And while they thought to lie hid in their obscure sins, they were scattered under a dark veil of forgetfulness, being horribly afraid and troubled with exceeding great astonishment. For neither did the den that held them, keep them from fear: for noises coming down troubled them, and sad visions appearing to them, affrighted them. And no power of fire could give them light, neither could the bright flames of the stars enlighten that horrible night" (Wisdom 17:3-5). 


"But they that during that night, in which nothing could be done, and which came upon them from the lowest and deepest hell, slept the same sleep. Were sometimes molested with the fear of monsters, sometimes fainted away, their soul failing them: for a sudden and unlooked for fear was come upon them" (Wisdom 17:13-14). 


"For if any one were a husbandman, or a shepherd, or a labourer in the field, and was suddenly overtaken, he endured a necessity from which he could not fly" (Wisdom 17:16).


It is intriguing to see the connections between these excerpts and the prophecies of the Three Days of Darkness, but it makes sense if this is supposed to be a similar event. However, I think it is certainly possible that most of the fake prophecies that are associated with the Three Days simply paraphrased this description.