The pathos is the diagnostic trait. You can trace the form through ragtime and chain gang chants into African tribal traditions, but the form won't tell you the blues. The traditional 8-bar and 12-bar blues forms, usually played in E or A, are completely set aside in some of the greatest cuts from Bessie Smith's Barrelhouse Blues number,for example; but there is no mistaking the down-deep protest against pain that is the real earmark of blues.

One couplet says the blues ain't nothin but a low down dirty shame. Another says it is a ten-dollar woman hooked up with a two dollar man. You won't get far defining it in any cut and dried way, because it is a hot, volatile, shifting voice of deep feeling. But, however this occurs, it is always the blues. That underlies all the musicological parsing and gives you the deeper grasp of what it is. Then you'll see it in thousands of forms from gospel to rock to Coltrane, unmistakably.