out of the blue??

Above answers are totally right about "out of the blue".
I just want to say something about "mentioned" & "affair" these 2 words from the original sentence.
1) he just mentioned the affair out of the blue. 如解做 '他突然間提及這件事' 是不需用 just 這個adverb的.
2) mentioned = 提及. 但在此解做 admitted承認.
3) affair = 事件. 但在此解做 婚外情. (正如 christy.kwan所說). 如果是指事件的話, 則應用 case 而不是 affair.
故此句的意思是: 他突然間承認了他有婚外情 (外遇).
Please correct me if i'm wrong. Tks.

out of the blue 的意思是 "特然從天上掉下來的", 那麼也可是: 忽然 (suddenly) 或 意外地 (unexpectedly)

在 yourdictionary 也可找到 (http://www.yourdictionary.com/blue)
out of the blue:
as if from the sky; without being expected or foreseen

For example: Out of the blue, the martial art champion is a small girl.
很意外地, 武術冠軍是個個子小小的女孩.
也可寫成: Unexpectedly, the martial art champion is a small girl.

2009-02-06 00:27:30 補充：
原句"he just mentioned the affair out of the blue"
原句的意思是: 他忽然間提及那事 or 他突然說起那事 :-)
當心那個affair的意思, 在美國口語中, affair可以譯為"婚外情","移情別戀"等意思. 所以要小心用這個字.

out of the blue = 意外地 / 出人意外地
mentioned = 提及
the affair = 事件

全句係.... 他剛剛意外地提及事件