The last two bars is what is known as a "turnaround". You turn the chords around to go back to the first bar.
This is commonly called the 12 bar blues.
It's usually three lines. The first line is stated in the singing. The second line is repeated so that the blue singer can have time to think of the third line. Then the third line ends with a rhyme for the first two lines.
The problem is that most blues players in the tradition will not always stick to it. The may play an 11 and a half bar blues or a thirteen bar blues. It's amazing but they stay together in playing this.
The note customarilly bent to make the guitar cry is the seventh degree of the scale. Sometimes you take the sixth note of the scale and bend it up to the seventh by pulling the guitar string sideways. Sometimes you take the seventh note of the scale and bend it up a half step. There are various ways to do this. But the important thing to remember is that the seventh is really the flatted seventh and not the actual seventh note of a major scale.
The blues scale looks something like this.
1,2,b3,4,b5,5,b7,7 and 8. The b2 is rarely used. The 6th note of the scale is commonly used but the b3,b5 and the b7 are the essential blues notes of the scale that give the "mode" it's color.
Sometimes the b6 is used if you bend it upward from the 5. (These numbers represent scale tones).
The Delta blues is the langorous, slow blues that sometimes has the feeling of flamenco singing. (The so-called "deep song" or cante hondo.) This is different than the more "ragtime" sounding Piedmont blues.
Son House, John Lee Hooker, Lightning Hopkins, B.B. King are close to the Delta tradition. Mississippi John Hurt, Brownie McGhee, Furry Lewis, would be more in the Piedmont tradition but many blues artist can do both and cross over such as Big Bill Broonzy.
As to the controversy regarding whether blues musicians bother with theory, it depends on how you define the blues. If you have a narrow definition of it, then the more "country" or "folk" aspects are intuitive and not theoretical. But if you allow for the blues to be broader based such as found in early jazz, swing or even Charlie Parker in Be-bop, then some theory has to be used. One comment about theory though that musicians often forget. It often comes after the fact. A great musician comes along and plays something and then someone makes up a theory about it.