人的情绪时起时伏，feel blue 的意思是：be depressed or sad 心情闷闷不乐，情绪郁郁寡欢。
例句：I woke up feeling blue and I've been feeling blue all day. I don't know why: everything's okay with me. Maybe it's this terrible weather--cold and rainy for five whole days in a row!
我一早醒来就心情不好，接着一整天都闷闷不乐。我都不知道是什么缘故，因为我的一切情况都正常。也许是由于这讨厌的气候；整整一连五天都是没完没了的阴雨天气！