#1
So, a "typical" blues rhythm part contains a pattern something like this:
------------------- 
------------------- 
------------------- 
-2--2-4--4-2--2-4--
-0--0-0--0-0--0-0--
-------------------

From what I understand, those are root-fifth and root-sixth diads. I've heard this in a bunch of blues songs. However, the "blues scale" does not contain a 6th degree, yet it is commonly used for soloing over such a passage.

How come the 6th is used in the rhythm part, but never in the lead part?

Thanks in advance. Please excuse me for being vague.
#2
What do you mean with never used? The pentatonic major is:

1 2 3 5 6 or A pentatonic major is A B C# E F#
#3
It is using the dominant form of the key (switching from major to to minor) It can be in the lead and I play it a lot. In A, play

--------------------------------------------
-----------5--5--7b---------------7--------
-----------5--5--7b--5b--5h6-------------
-------------------------------------------
----------------------------------------7---

You could play that at the end of a 12 bar to go into the next one.
I am the only sane person on the planet. Does that make me crazy?

Crank the Mids
#4
The blues scale is rarely used harmonically. Typically you'd use it to make a melody over a dominant chord, which contains a Maj3 which is absent from the blues scale