This w/u is concerned with pre-1930's blues only. Most of its facts and musings can be applied to later days, but don't stake your life on it.
One thing we could say which sets the blues apart from its brothers of rock and roll, punk, jazz, and funk, is its simplicity in both conception and evolution. Blues men, and just as much the blues women, do not wish to present you with a complicated art. Their treasure in music is not within finding original ways to display what they've got to say. Though this may happen on its own, it is not the primary concern. Complication is an unneeded lettuce. Blues, through and through, is all about digging closer and closer in your own heart with the simplest spade you can find. It gives the notion that you can find progression and deeper meaning in your art just by meaning what you've got to say, really bad.
With most blues songs you've got a very simple verse syntax. The first line states, the second repeats, and the third elaborates in some way. And in order to really get it (which shouldn't be hard at all -- when you're ready), instead of listening for clever lines, or twitches in the chord progression, or hip hop whistles sounding off every beat, just listen to what the man had to say. What you're doing is growing two feet shorter, getting skinny and hungry, becoming black and poor and spit on, going down south, falling over train tracks, getting up and visiting your father in jail. You see him in there and he's skinnier. Skinnier 'n the bits of bacon and ham we used to get, worser than the worse days he'd just eaten bread with dust. Another tooth is missing. Get to wonderin', walking back to your house many miles away in the dark, after your father said nothing but cried at you and rubbed the top of your head, both your feet hurting,
i won'er why dey 'lectrocute a man at the one o'clock hour in night
and i won'er why dey 'lectrocute a man at the one o'clock hour O night!
b'cause the current is much stronger... den the folkses gone turned out all de lights
*
The words aren't about stress, aren't about some voice contest of screams and yawps, it's about getting it right. Getting it right with corn on your lips and singing because the reason in your gut is hungry for something.
So yeah, blues was about being oppressed. But it wasn't just about sadness- it was about companionship and heart-to-heart playing. It was about cheering at times. Blues was possibly the first time in free African American culture where their descriminations were used to uplift. A common view of the blacks back then were savage animals, horny and stupid. Blues changed the meaning of these words for blacks. Wild man blues, crazy sexy women from ghettos, the epitomy of life in a good gin and tonic. That was blues, that was theirs through slavery, theirs for centuries, and for their children.
The beat was important. Plenty of writeups already have gone on about it. But it's important to distinguish that blues could be a singer, or a band, and the difference was in whether the voice was by itself, or well backed up. Finding enough intruments to have a good band better than a hearfelt voice back in the day was rough stuff.
Many blues verses find themselves being traded around- this wasn't pegged as a copyright violation. You had your core verses, and around them deviated between singers and bands. Blues was an avenue of profit once records and bands become accepted, but all this hackneying was allowed because blues didn't forget the people; it never fully became of money (one of the few genres that didn't fall victim to big business). You couldn't peg down the first person to sing about being locked up or sent to work on the railroad after the Civil War anyway- the oppression was everywhere. When you sing about yourself in the blues, when you use a central verse or a lick and then go with it, you're singing your soul, and you're singing what's in others; souls too.
Ironically, the blues style was imitated by whites and became their entertainment, up until the 1900s, when whites began seeing slighter and slighter rises in their darker skin compadres, dropped the "nigger music" on them and targeted what they too enjoyed as devil's music. It wasn't until Mamie Smith recorded the very first blues record in 1920, "Crazy Blues," that blacks and whites slowly began to treasure the music their parents once showed them. And just after her wake, with the help of jazz becoming a more popular (but at the time less serious) style, nearly every blues band that could would record records.
* - Lyrics first recorded by Blind Lemon Jefferson.