Out of the blue

This is an informal idiom that describes an event that happened unexpectedly.
It is used as an adverb, and is very popular in spoken conversations.

So, the sentence:
“An old friend from school called me unexpectedly.”
could be said:
“An old friend from school called me out of the blue.”

The "blue" in the phrase refers to the sky.
You can imagine that anything, apart from rain, falling out of the blue sky would quite unexpected.

この表現は、予期していなかったことが起きたという意味のくだけた慣用句で、会話でとてもよく使われる表現です。

たとえば、
“An old friend from school called me unexpectedly.”
「むかしの友人が突然電話してきたんだ。」

この表現が下記のように表せます。
“An old friend from school called me out of the blue.”

このフレーズの中の"blue"というのは、空のことを指しています。
青空から雨以外のものが降ってきたら、予想外で驚きますよね。