It Wasn't An Eclipse
Eclipses are a natural phenomenon that occur when one celestial body obscures another. From earth, an eclipse of the sun (a solar eclipse) happens when the moon is perfectly in line between the earth and the sun (an eclipse of the sun is only possible at new moon), while an eclipse of the moon (a lunar eclipse) takes place when the earth is perfectly in line between the sun and the moon (i.e. an eclipse of the moon is only possible at full moon). An interesting coincidence (if it is just a coincidence) is that although the diameter of the sun is about 400 times that of the moon, the sun is also 400 times farther away from earth than the moon, thereby making the sun and moon appear the same size in the sky.
There are a number of possible references to eclipses in The Bible, but the sudden darkness at the time of the crucifixion of Jesus Christ is almost certainly not one of them:
"Now from the sixth hour there was darkness over all the land until the ninth hour. And about the ninth hour Jesus cried with a loud voice, "Eli, Eli, lama sabach-thani?" that is, "My God, My God, why hast Thou forsaken Me?" [see Eloi, Eloi, Lama Sabachthani?]
"And some of the bystanders hearing it said, "This man is calling Elijah."
"And one of them at once ran and took a sponge, filled it with vinegar, and put it on a reed, and gave it to Him to drink. But the others said, "Wait, let us see whether Elijah will come to save Him."
"And Jesus cried again with a loud voice and yielded up His Spirit."
"And behold, the curtain of the temple was torn in two, from top to bottom; and the earth shook, and the rocks were split; the tombs also were opened, and many bodies of the saints who had fallen asleep were raised, and coming out of the tombs after His resurrection they went into the holy city and appeared to many."
"When the centurion and those who were with Him, keeping watch over Jesus, saw the earthquake and what took place, they were filled with awe, and said, "Truly this was the Son of God!" (Matthew 27:45-54 RSV)
The theory that the darkness was caused by a natural solar eclipse is flawed by the fact that solar eclipses can only last for a few minutes, while the darkness at the crucifixion lasted about three hours. As well, it could not have been an eclipse of the sun because the Passover, at which time Christ was crucified, always was and is observed at the fourteenth of the month of Nisan, on the Hebrew calendar, which a lunar based calendar. There is always a full moon (when the earth is between the sun and the moon) at Passover, and solar eclipses can only happen at new moon (when the moon is between the sun and the earth), which is two weeks before or after the full moon.
In view of the other miraculous events that happened at the same time, it's reasonable to assume that the darkness too was brought about by God's will, rather than mere natural events of the heavens.
Fact Finder: Will there be great "signs in the heavens" just prior to The Return Of Jesus Christ?
See Signs In The Heavens