Bl. Anna Maria Taigi's three days of darkness prophecy is described on pp. 310 ff. and the Appendix of The life of the venerable Anna Maria Taigi (1873).

On the predicted future pontiff, she says he was/is(?) not of the Cardinals, "but that he was a humble priest not at that time in the Pontifical States, but in a very distant country."

She said that he [the future Pontiff] would be elected in an extraordinary manner; that he would introduce reforms; that, if men were grateful for them, the Lord would load them with blessings; but that, if they abased him, His all-powerful arm would inflict heavy chastisements upon them. She said that this Pontiff, chosen according to the Heart of God, would be assisted by Him with very special lights; that his name would be famous throughout the world, and applauded by the people; that the Turk himself would venerate him, and send to compliment him. She said that he was the holy Pontiff destined to bear the rage of the tempest which was to be let loose against the bark of Peter; that the arm of God would sustain him, and defend him against the impious, who should be humbled and confounded; that in the end he would have the gift of miracles, and that the Church, after painful vicissitudes, would obtain so glorious a triumph, that the world would be astounded.

The preaching of Sts. Peter and Paul, that "St. Michael, appearing on earth, shall chain up Satan until the times of the preaching of Antichrist,"  that "Religion will everywhere extend its empire," that "Russia will be converted, as will also England and China," and that "all nations will rejoice in contemplating this splendid triumph of the Church" is reported by P. Calixte, described in the appendix.
"P. Calixte's work was approved by his own Superior, the General of the Trinitarians and Postulator of her cause." (p. 316).

"This scourge, he [P. Raffaele Natali] told many persons, was to be a supernatural darkness, which was to prevail for three days, during which blessed candles would alone give light." (p. 314).

It will already have been observed…that during several consecutive days Anna Maria saw the world enveloped in a dense and awful darkness, accompanied by the falling of walls and timber, as if a great edifice were crumbling into fragments. … Her confidant [P. Raffaele Natali] must have had the best opportunities of knowing her mind and hearing her explanations, and,indeed, the mention of blessed candles alone giving light seems to show that the darkness foretold is to be real and sensible, not figurative.

"Despise not prophecies. But prove all things; hold fast that which is good." —St. Paul, 1 Thessalonians 5:20-21