Now from the sixth hour there was darkness over all the land until the ninth hour. (ASV)
This mysterious darkness was seen by nobody outside of the New Testament, and was not recorded even by such prolific historians as Philo (around 50 at that time), Pliny the Elder (around 10 at that time), or Josephus (born around that time). In particular, Pliny's Natural History is full of anecdotes at least as bizarre as three hours of mysterious darkness and zombies talking walks from their tombs.
And how big was this darkened area supposed to be? If it was just the Jerusalem area, then it could easily have escaped Pliny's notice, though not that of Philo or Josephus. But if it was also over the rest of the Mediterranean area, then Pliny would have seen it from his boyhood home near Lake Como, Italy. And even if he had not paid much attention to it, his elders would have noticed it.
And many other historians would have either noticed that event or written about it. But they didn't.
Edit this section if you doubt error.
See Richard Carrier's paper Thallus: an Analysis at Internet Infidels for a discussion about Thallus, "a pagan chronologer of unknown date who is occasionally mentioned in the works of Christian apologists, modern and ancient, as a 1st century pagan witness to the gospel tradition of a "darkness" at the death of Christ".
See also Glenn Miller's paper Extrabiblical Witnesses to Jesus before 200 a.d. -- Thallus at Christian Thinktank.