Blues was usually written about hardships of the citizen, oppression, women, money, drugs, alcohol, you name it. Blues then developed to become the early stages of rock and roll.
Above is Jimi Hendrix, he is named one of the best guitarists ever to set foot on this planet. We listened to his track 'Red House' which is in formulae, a blues song.
To the right is BB King, He is a legendary guitarist. We listened to his blues track 'Three O'clock Blues' which is in the 12 bar structure.
Left is the blues guitarist John Lee Hooker, again a legendary guitarist. We listened to his track 'Boom Boom' which isn't actually in the 12 bar structure, it is a 12 bar broken up to fit the staccato structure of the piece.