When I'm soloing over the 12 bar blues in A (A7 D9 E9) I only use notes from the minor pentatonic scale in A.
I know it should be possible to use the major pentatonic and even other scales. But I have no idea what scales and when they are appropriate to use.
All the videos and lessons I find never provide any answer to these questions. Is there any reference material or guidelines that I can follow to spice up my licks?
There's one major concept missing from this discussion: What are your ears hearing you play over this blues progression? When you play/practice this blues are you able to hear a melody or theme that you would like to be able to play from your instrument? If not, who do you listen to that would play what you want to hear? Understanding scales and their relationship to the chords is very important but if you focus on the mechanics too much you forget that what you are trying to do is play music.
As far as the mechanics are concerned, assuming you understand basic music theory, you should be able to analyze a chord progression and determine which scale fits each chord. That having been said, your original question was about playing over a blues progression so let's assume you have analyzed the 12 bar blues. Notice the I7 chord (A7 in your example) is really a V7 chord meaning the actual scale it was derived from (again using A7 for this example) is the D Major (ionian mode) scale. A7 is the diatonic V7 chord in D major and therefore would use the D major scale as it's chord scale. We know that the mode of the V7 chord is Mixolydian and if you understand modes and their structure you will know that the D Major / Ionian mode is the same set of 7 notes as the A Mixolydian mode.
In your blues, the next chord after the I7 is the IV7 chord (in the A blues the IV7 chord is D7). Again, if you know that D7 is borrowed from the key of G Major (D7 is the V chord in G, G Major/ Ionian mode so the D Mixolydian mode would be the appropriate chord scale in this case. Your blues then goes back to the I7 chord for 2 bars, the IV7 chord gets 2 bars, then the I7 chord gets 2 more bars.
We are now approaching the last 4 bars of a 12 bar blues, the critical point! Some call it a turn-around, but it takes you to the V7 chord which is E7. E is the V chord in A major therefore the A major scale/Ionian mode and E Mixolydian would be the scale to use. You mention using a pentatonic scale so you should know that in most configurations of the pentatonic scale the 5 notes fall within the Mixolydian mode as well.
Notice that this whole structure is based on a V7 to I resolution. All V7 chords want to resolve to the I major 7 chord by nature and if you construct a melody that targets this resolution somehow, you might actually be playing music. Listen to Stevie Ray Vaughn or Buddy Guy for guitar blues. Charlie Parker and Sonny Rollins use the same 12 bar blues structure but in a bebop style. Doesn't matter. 12 bar blues is 12 bar blues whether it swings or rocks. After all, there are only 12 notes, what you do with them will define your relationship with music.
Use the notes from A pent min. and A pent maj. while the progression is on A. You may even use the min. and maj. blues scale notes - one extra for each. These obviously work over D and E (you already use them!) but better to use the pents or blues of D over the D bars and E over the E bars. We have discussed similar in other answers. Basically there are about 3 notes which don't theoretically fit , (left from the 12 chromatics), but they still get used, mainly as passing notes.
As you've already found out, the A minor pentatonic scale - or, if you add the flat 5 = Eb, the A blues scale - fits all three chords. But your solos can indeed be made more interesting if you add more notes. You can use chord tones to spice up your solo and to incorporate the changes into your melodies.
Below are the chord tones that are not already part of the A blues scale and that can be added when the corresponding chord is played:
A7: C#
D7: F#
E7: B, G#
Of course, all chords can be played as ninth chords, which gives you even more options: B for A9, and F# for E9 (the E of D9 is already part of the original A blues scale).
This is a first step which will help you to make your solos more interesting, and which will make your melodies reflect the underlying progression.
One problem I've noticed with my students is that when using chord tones, you always need to know/hear where exactly in the progression you are. E.g., the note C# can be played as a target note over the A7 chord, but not (as a target note) over D7, because then it would clash with the 7th of the chord (the C). (By the way, note that any note is fine as a passing tone). This means that now you need to listen even more carefully and you need to be fully aware of the changes. This is a very good thing anyway, because it will result in more musical solos. However, since exclusively using the blues scale doesn't force you to exactly hear the changes (because all notes fit more or less all the time), many beginners are not very much aware of the changes.
You can use many more extra notes and scales, but I think the above is a very good starting point. Using the major pentatonic scale results in almost the same. Also here you can't use one scale over all three chords, for the same reasons as above. Over A7 you can use the A major pentatonic scale, but it contains the C#, which, as mentioned above, will clash with the note C of the D7 chord. One way to use the major pentatonic scale over a blues is to switch scales for every chord, i.e. use A major pent. over A7, D major pent. over D7, and E major pent. over E7. But this is just one option and not the only possibility.
Only if you feel comfortable with all of the above, you might want to try other things; they will generally sound more jazzy. One option would be the diminished scale (see this answer), or any other scale that can be used over dominant seventh chords.
To hear some weird sounds over the blues, you can check out this instructive example by guitarist Oz Noy. He demonstrates different scales such as the standard minor and major pentatonic scales and the mixolydian scale, but also more oddly sounding ones like the whole tone, the diminished and the altered scale.
Two good references:
Blues Scales: Essential Tools for Jazz Improvisation. by Dan Greenblatt
Tony Monaco's online "Blues" video lessons.
A very important thing for you to realize is: pick any scale (pentatonic or otherwise, blues or otherwise). As long as you create a logical, meaningful solo in that scale, it will work, and the scale doesn't have to exactly fit the key you're in. It doesn't need to match the background chords. It just needs to stand as a good solo on its own.
If you listen to some organ work by Jimmy Smith or Jimmy McDuff, sometimes their solos are pretty simple blues stuff. But, they will choose a different blues scale. So on the first pass, they might do a F minor blues. But on the second pass they might do an F major blues, even though the chords haven't changed. They'll pick one and stick with it, so that the solo sounds logical.
