So, a "typical" blues rhythm part contains a pattern something like this:

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.
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#
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


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
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