The meaning of blues is born in the Elizabethan Age: "To Have the blues devils", was a common phrase meaning dispair and sorrow. First blues (Rural Blues) waere based on a structure of 12 measures, divided in 3 parts of 4 measures each. The first part, on a Root note, sung of Protest, sex, sorrow or rage. The second part was about the same concepts, on a 4thdegree chord. The last part, on the 5thdegree was about hope or rebellion for segregation.

This is the F Rural blues standard structure.

It's a 12 bar track: first 4 are in F7, as you can see and many of the changes in next pages focus on them.
Measures 1 5 and 9 are the "blues essential" that's keeping the 1st7 degree in measure 1, 4th7 degree in measure 5 and 5th7 in measure 9.
That's not actually correct: go to next page to discover why.