英语里为什么用blue表示难过？

When you are sad, you can say "feel blue". 心里难受的时候，英语说feel blue。 Blue is just a color, so why the sadness? 蓝色就是个颜色，咋跟难过挂上了钩？ Long ago, "blue" started to mean a low mood. 很久以前，blue就有了情绪低落的意思。 People even called sadness "the blues". 人们干脆把难过叫做the blues。 Sad songs are still called blues music. 现在的伤感音乐，还叫blues呢。 So blue slowly became the color of sadness. 蓝色就这么慢慢成了忧郁的颜色。 Next time you feel blue, you know why. 下次你feel blue，就知道为啥啦。