The Blues Curl on the Flat 3rd
The Blues Curl is a 1/4 tone bend that happens (pretty much) only on the Flat 3rd scale degree - in a Blues in A, it will be the note C that gets this treatment. It's only a subtle movement but it's super cool.
I played the Blues Curl many many years before I even thought about how or why it worked... and probably many of the greatest Blues guitarists never gave it a second thought.
But the 'theory' of blues is interesting in that is shouldn't really work.
Playing the A Minor Pentatonic (A C D E G) over an A7 Chord (A C# E G) you can see that the C and D notes should clash with the C# in the chord - and they do - but it's the sound of the blues - it shouldn't work but it does. And remember the biggest rule of all: If It Sounds Good It Is Good!
The Curl bends the Flat 3 (the note C in the Key of A) slightly sharp - roughly a 1/4 tone (half a semitone), pushing it closer to the Major 3rd (the C# in the the A7 chord).
But please don't be concerned too much with the theory - it's very much a practical thing to be doing (but you can check out my ebook Practical Music Theory if you're keen to understand it all!).
Remember that listening to great Blues is an essential part of the course, so try and check out all my recommended albums, they're the best of the best!
B.B. King - Live in Cook County Jail (around 1965)
We could have has a lot of BB on this list!! The Regal album is considered the best by most people, but this is a close second, really great sound on this album and man he tears it up, he is the Chairman Of The Board!!
Here's a sample backing track from my Jam Blues II collection, a Blues in A in the style of BB King. You can download and jam over it and if you enjoy it, please consider buying the rest of the album, they're all great fun for jamming!
You could practice playing your scale up and down over the backing track - will help you understand the quality of each note of the scale, and help you develop your timing and groove. Once you got it down you might like to try mixing the notes up a bit too!