In the 18th century the English phrase blue devils referred to delirium tremens, the upsetting hallucinations brought on by severe alcohol withdrawal. This was later shortened to the blues, which described states of depression and upset, and it was eventually adopted as the name for the melancholic songs that the musical genre encompasses.
In addition, the characteristic flatted or “bent” notes of the scale used in blues, which give the music its plaintive sound, are referred to as “blue notes.”