These things confuse the hell out of me. Whenever I'm learning a blues tune these always make the least sense theory wise to me.

I understand we're trying to resolve to the I chord and good way of doing this is by getting to the V or ii chord. For example in an old delta blues tune I recently learned by Pink Anderson called Everyday of the week (G major blues), the turnaround starts on G-E7-A7-D7-G. Essentially a vi-ii-V-I. Here's the link to the lesson:

What I don't get are the descending melodic elements to the turnaround especially after the last G (the B-D-A#-C#-A-C-G-B-G line). I'm really trying to understand why these work even though a couple of the notes aren't in the scale). To me it looks like the A# is from the C7 chord and the C# is from the A7 chord.

I may have just answered my own question and I know how a lot of times in blues tunes they just use 7 chords. Why is that? Just breaking the rules to get a good color?

I guess I need some guidance in how to understand blues music.