Out of a clear sky 的用法

Out of the blue
Out of a/the clear blue sky
Out of a/the clear sky
都是一样的意思：
without advance notice or warning; if something happens out of the blue,it happens suddenly and you are not expecting it:
examples:
An old classmate phoned her out of a clear sky.她的老同学突然打电话个她
"Why did she do that?"
"I have no idea.It was completely out of the blue."
"Did you know they would move you to a different project?"
"Not at all.It was completely out of the blue."