According to the National Weather Service, a "bolt from the blue" is a cloud-to-ground lightning strike that appears to come ...

This is an incredible picture of a “bolt from the blue” lightning strike from downtown Houston Tuesday. Quentin Dunn sent this to me from his office with downtown in the distance.

According to the National Weather Service, a bolt from the blue is a cloud to ground lightning strike which appears to come out of a clear sky, hence the name.