@dudelove
Well actually everyday life as a black person in the States in that period was kind of sad, but their music's purpose was to uplift the community. Almost all music eg. Swing, Gospel, Blues, Negro Spirituals, that either originated from the slavery era or evolved out of it tended to reflect a kind of defiance against the horrific oppression that they had to endure. Not so much in the violent or raging sense, but more of a refusal to be robbed of their dignity regardless of the circumstance.
In that sense, it's really that deep seated sadness and sense of injustice in slave culture that gives the blues its characteristic mix of melancholy and joy. Minor over Major, so to speak.
Blues "form" is another thing altogether, you can write something in a blues chord progression that sounds nothing like the blues, it's just a musical form and if abused has no real meaning in the "Blues" sense, but can still be a great piece of music.