From the given options, select the one that rightly explains the given expression:
A bolt from the blue

An unexpected and sudden event

The idiom "a bolt from the blue" means an unexpected and sudden event. This idiom is often used to describe a piece of news that is very surprising or shocking. The origin of this idiom is unknown, but it is thought to come from the idea of a bolt of lightning coming out of a clear sky.

The other options are incorrect. "A delayed event" is not an unexpected event. "A very pleasant but bluish event" is not a common idiom, and it does not mean an unexpected event. "Long attainted event" means an event that took a long time to achieve, which is not the same as an unexpected event.