If something happens out of the blue, it happens suddenly and you are not expecting it.
At a completely unexpected time.
An event that occurs unexpectedly, without any warning or preparation.
1. I met my childhood friend, Jamie out of the blue.
2. It started to rain, out of the blue.
3. My friends surprised me with a small party, out of the blue.
4. There was a serious accident at the 4-way stop that happened out of the blue.
not predicted, surprising, unanticipated, unexpected, unforeseen, unplanned, unprepared for