Can anyone recommend any good resources (books or online) for learning the Blues on the guitar? Scales and theory is what I am looking for.
You can find backing tracks online. Get a few(or as many as you want) and start soloing. Learn the progression. The basic blues is a shuffle rhythm around the dominant versions of the primary chords in key. The 12-bar blues follows the pattern 4*I7 2*IV7 2*I7 V7 IV7 2*I7.
e.g., A7 - - - D7 - A7 - E7 D7 A7 - repeat. The last 4 chords are called the turn-around.
Learn as many turn-around licks as you can so that you can get an authentic feel in your solos.
When soloing over the the first 8 or 9 chords you will use the minor pentatonic with added b5 and sometimes the M3rd and M6th called the blues scale
You will change scales on the D7 and E7 chord but the actual note material generally stays the same. e.g, you can use the A blues scale on the D7 and E7 chords but it's better to use the D blues scale and E blues scale resp. This is really just a shift in thinking because the scales
A C D (D#) E G
D F G (F#) A C
E G A (A#) B D
Have all the same notes (from the A natural minor scale)
A C D E G
A C D F G (D Blues starting on A)
A B D E G (E Blues starting on A)
except for some minor alterations. But emphasizing these alterations is what helps one hear the changes which means sometimes adding the major third and sometimes not and using the B on the E chord and not on the A.
Learning the rhythm is very important. You can't learn to run before you learn to walk. Try learning the rhythm aspects but through in a little lead fills. Learn my flavors of blues rhythm as you can.
Listen to a lot of the blues masters to see how they did it. Experiment with alterations of the scale once you learn the basics well. Add some chromaticism, some minor blues, etc...
Beyond that detailed answer, look up a few songs on Youtube if you don't have 'em already. "Boogie Chillun" by John Lee Hooker, "Death Letter Blues" by Son House, Blind Boy Fuller's "Rag Mama Rag", "Dust My Broom" by Elmore James, "Stormy Monday Blues" by T-Bone Walker but perhaps the Allman Brothers Band. It's all blues by some of the fundamental blues artists, but we're talking miles and miles from each other in terms of ... everything.
"Boogie Chillun" basically sits on one chord from beginning to end. "Rag Mama Rag" is a rag, and it's C A D G over and over. "Death Letter" is a bottleneck open-tuning solo work. With "Dust My Broom" we have full-band blues but the chords are straight 12-bar. By the time you get to "Stormy Monday", you have very sophisticated changes. It's all blues, it's all related, but if you're wanting to learn one style, you'll have to pick up entirely different sets of knowledge.
At the middle ground between primal one-chord blues and sophisticated is the 12-bar mentioned by Abstract. There's variations, but that's a good place to start.
For any particular key, the basic scale is root, flat third, fourth, flat five, fifth, and the flat or dominant seventh. In A, we're talking A, C, D Eb, E and G. Except, the C you're looking for is a bit sharp from the standard C, but not as sharp as C#. Listen to "Spoonful" and you'll hear that note. String bending started so you can get those notes.
There's a guy on Youtube, Adam Gussow. He played harp in Satan and Adam, and he started to create video after video of blues harp instruction. A lot is harp-specific, talking about tongue blocking and crossharp and etc. etc. But there's a lot on just how you break down songs, what the 12-bar blues and the blues scale are, and more. Well worth your time to start at the beginning. And beyond that, listen to the blues and try to play along.
There's a very good YouTube based instructor named Justin Sandercoe. He has a huge collection of lessons and resources available including many that are focused on the blues. All of his material is free, though there is some available for purchase.
I've found the Blues courses at Truefire to be fantastic. www.truefire.com
No affiliation with them - just a happy customer. You can get courses which delve right into Blues theory, or just get a '50 Blues licks' set to learn some great licks.
I have a preference for lessons by Jeff McErlain and Robbie Calvo from there, but that is just me. You may find some of the other instructors to be better for you to learn from.
If you have an iPad, they have a great (free) iPad app called Guitar Lab which you can see a small subset of their lessons for free which change each month.
In the few short months that I have been using their lessons, my Blues playing and understanding has improved 100%.
I always suggest going back and studying the source material; the original masters. YouTube has literally hundreds of fine videos and recordings of all the old blues greats. And each one you find will give you links to more. Listen. Blues is not about theory and scales, it's about emotion and performance. Most all the old bluesmen were self-taught musicians who wouldn't know a mode from a diminished chord. They learned by watching and listening to other players.
The tunes and artists listed by VarLogRant above would be a good start.