William Lane Craig recently wrote about some corroboration of the darkness at the time of Jesus' crucifixion (Mark 15:33) from an early non-Christian source, Thallus. Craig makes some good points, but I want to address some issues that aren't often discussed when this subject comes up.
In the past, I've recommended a few online resources that discuss the darkness: here, here, and here. As you can see from reading those sources, the dating of Thallus' writing is disputed. Craig comments that "most scholars date Thallus’ History to the mid-first century, that is, sometime around AD 50". The most significant piece of evidence I'm aware of on the dating issue is Tertullian's comments in the following passage:
"The histories of the most ancient nations, such as the Egyptians, the Chaldeans, the Phoenicians, would need to be ransacked; the men of these various nations who have information to give, would have to be called in as witnesses. Manetho the Egyptian, and Berosus the Chaldean, and Hieromus the Phoenician king of Tyre; their successors too, Ptolemy the Mendesian, and Demetrius Phalereus, and King Juba, and Apion, and Thallus, and their critic the Jew Josephus, the native vindicator of the ancient history of his people, who either authenticates or refutes the others." (Apology, 19)
Tertullian seems to be referring to a historian by the name of Thallus who was criticized by Josephus. Presumably, then, Thallus wrote no later than the late first century. While it would be possible to interpret Tertullian as referring to Josephus' criticism of concepts that would later be advocated by Thallus, or to conclude that Tertullian was mistaken about the timing, the most natural way to take his comments is to conclude that Thallus wrote prior to Josephus. The reference to Apion, who's mentioned just before Thallus, probably was intended by Tertullian to be taken that way. At a minimum, Thallus wrote prior to when Theophilus of Antioch mentions him around the year 180 A.D. (To Autolychus, 3:29) He probably wrote prior to Josephus, as Tertullian suggests, which would place him in the mid to late first century. (Thallus' reference to the darkness at the time of the crucifixion tells us that he couldn't have written earlier.)
The history written by Thallus is no longer extant. We're told about Thallus' comments by a third-century Christian, Julius Africanus. Craig focuses on the testimony of Julius Africanus in his article. What isn't often noted is that other ancient sources also suggest that the darkness at the crucifixion was acknowledged by non-Christians.
Tertullian writes that "You [Romans] yourselves have the account of the world-portent still in your archives." (Apology, 21) It's common to suggest that Tertullian was only assuming that the event would be mentioned in the Roman archives. He didn't see any such record himself. He seems to have been mistaken when he referred to Roman records elsewhere. But even if we assume that Tertullian was mistaken in this instance, it would still be significant that he expected non-Christian corroboration.
Later, Jerome commented:
"Those who have written against the Gospels suspect that Christ's disciples, through ignorance, have interpreted an eclipse of the sun in connection with the Lord's Resurrection." (Thomas Scheck, trans., St. Jerome: Commentary On Matthew [Washington, D.C.: The Catholic University Of America Press, 2008], p. 318)
Notice the plural ("those"). And note the term "interpreted". It seems that these critics of the gospels were disputing issues like the source and implications of the darkness, not its existence or timing. Jerome goes on, just afterward, to argue against such a view that would accept the existence and timing of the darkness, but dispute its source or implications (ibid., pp. 318-319). And that's the view Julius Africanus was responding to. Apparently, it was a common position among Christianity's critics.
The only early non-Christian source I'm aware of who disputes the historicity of the darkness is Celsus, who wrote in the late second century. Origen comments, "He [Celsus] imagines also that both the earthquake and the darkness were an invention" (Against Celsus, 2:59). What Celsus "imagines", however, doesn't carry much weight. Earlier and better sources disagreed with him, and the testimony of Tertullian, Julius Africanus, and Jerome suggests that it was much more common for non-Christians to acknowledge the event rather than deny it. Celsus was often unreasonable. See, for example, his attempts to dismiss the testimony of the resurrection witnesses later in the same section of Origen's treatise. Elsewhere (2:33), Origen notes that Celsus' dismissal of events like the darkness at the crucifixion, while accepting other events in the gospels, is based on what Celsus wants to believe, not any objective criterion.
The darkness seems to have been recorded by four first-century sources that we know of today, three Christian (Matthew, Mark, and Luke) and one non-Christian (Thallus). Though Celsus cast doubt on the historicity of the event, we aren't aware of any good argument he had to support that conclusion, and the more common non-Christian position seems to have been to accept the historicity of the event and explain it naturalistically.
Well might the sun in darkness hide
And shut his glories in,
When Christ, the mighty Maker died,
For man the creature’s sin.
(Isaac Watts, Alas! And Did My Savior Bleed?)