While I am unable to confirm the relation of the three days of
darkness with the fatima from any official source (CMIIW), three
days of darkness has been mentioned quite often by Catholic mistic
saints throughout the ages. The time also cannot be confirmed.
While the Church does not oblige us to believe prophecies from
the mistic saints, the Church does recognized the holiness of the
mistic saints. This is good enough for me for not taking it
lightly.
That is my opinion.