"Bolt from the blue" is an English idiom that refers to something that occurs out of nowhere and is a complete surprise. Something described in this manner can be a good or bad thing, but it must be something unexpected to merit the usage of this phrase. The meaning of "bolt from the blue" is derived from what would happen if a lightning bolt were to strike in a clear, blue sky. This would be quite unexpected, just as anything described in this manner would be surprising to those who experience it.
Idioms are phrases which take their meaning not from the literal definitions of their words or from their origins. Instead, their meanings evolve as they are used in a culture. They often gain unique meanings that are quite different than any literal interpretation, and they are used by speakers to spice up speech with colloquial color. Many of these idiomatic phrases are used to express surprise. One of the most popular of these expressions is the phrase "bolt from the blue."
It is important to realize that this phrase can be utilized in many different contexts. Many times, people use it to describe something that is regarded as a stroke of good luck. Sometimes it may often refer to a sudden reversal of fortune for the person speaking. As an example, someone might say, "Getting that job interview so soon after being laid off was like a bolt from the blue." The implication there is that the new job offer was completely unexpected by the speaker.
Unfortunately, there are some occasions that may warrant the phrase to describe some negative things occurring unexpectedly. Many times, it seems like bad news comes out of nowhere, or, to use an idiom similar to this one, "out of the blue." On those occasions, this popular phrase may be used to describe the shock of the bad tidings for the speaker or for the person he or she is describing. Consider the sentence, "He was in such good health and he got sick just like that; it was a real bolt from the blue."
This is one idiom where the meaning might be guessed simply by thinking about the phrase. A "bolt" is a reference to a lightning bolt, which would normally occur in the middle of a cloudy sky and a rainstorm. By contrast, a "bolt from the blue" suggests that the lightning is coming from a blue sky, which would likely be extremely surprising to all those who see it.