I cant believe that people get this so wrong, the 12 bar blues is not one riff 
it is a chord sequencehere u go:

For example if i were to play the 12 bar blues in C, the notes in the c major scale 
are
C D E F G A B C the chord sequence for the 12 bar blues is the 1st note the 4th note 
and the 5th note in that scale.
   So we would play C F and G.

We play a riff in the C major scale 4 times
Then we play a riff in the F scale  2 times
Then we play a riff in the C sclae  2 times
Then we play a riff in the G scale  1 time
Then we play a riff in the F scale  1 time
Then we play a riff in the C scale  1 time
Then we play a riff in the G scale  1 time

Which adds up to 12. get it 12 bar blues.

  so lets do a riff in the 12 bar blues in the key of C

C

G——————————————————————————
D—————————————2—2———5—2————  x4
A——3—3——5—5———————————————— 
E——————————————————————————

F

G——————————————————————————
D—————————————————————————— x2
A—————————————0—0———0—2————
E—1—1———3—3————————————————

C

G——————————————————————————
D—————————————2—2———5—2————  x2
A——3—3——5—5———————————————— 
E——————————————————————————

G

G——————————————————————————
D—————————————————————————— x1
A—————————————2—2———5—2————
E—3—3———5—5————————————————

F

G——————————————————————————
D—————————————————————————— x1
A—————————————0—0———0—2————
E—1—1———3—3————————————————

C

G——————————————————————————
D—————————————2—2———5—2————  x1
A——3—3——5—5———————————————— 
E——————————————————————————

G

G——————————————————————————
D—————————————————————————— x1
A—————————————2—2———5—2————
E—3—3———5—5————————————————

You can put any riff you want to that chord sequence but as long as you use the 
1st note of the scale, 4th note of the scale and the 5th note of the scale.

Enjoy.