This is a very simple, yet effective trick you can use to great effect, particularly while playing over blues changes. ( I – IV – V ) And yes, it's exactly what it sounds like – taking the 6 notes of the minor blues scale ( 1 - flat3 – 4 – flat5 – 5 – flat 7 ) and then adding the natural 3.
This creates an interesting, smooth, blurring effect over dominant chords because it highlights the major/minor juxtaposition that's one of the hallmarks of the blues. You should learn how to add the 3rd in multiple octaves to all 5 positions of the minor blues scale. Beginning with minor blues box 1, if, for example, you're in the key of A, you play the minor blues scale at fret 5. You'll add the 4th fret of the A string ( C# ) and then in the next octave add the C# on the G string at fret 6 and it's a simple as that.
You can follow the I, IV and V chords with this scale as they go past and it works great over all three chords. You'll be amazed at how much the addition of one little note can enhance your playing!
To familiarize your ear with the sound, I suggest you lay down a 12 bar I – IV – V blues on your looper pedal or play over a jam track in either G or A and first play the minor blues scale with the major 3rd rooted on the 6th string and follow the chords as the progress. Then once you're good at that, move the IV and V chord related scales up to the 5th string to consolidate the parameters of your scales.
Finally a really cool thing you can do in several keys, especially G, A, C, D and E, is to augment this idea of playing minor blues scales with added 3rds with open strings. For example, if you're in the key of A, when you're on the I chord ( A7 ), all of the open strings work well as toggle notes. ( E is the 5th, A is the root, D is the 4th, G is the flat7th, B is the 9th ) Then on the IV chord ( D7 ) you have: E is the 9th, A is the 5th, D is the root, G is the 4th, B is the 6th and finally for the V chord ( E7 ) E is the root, A is the 4th, D is the flat7th, G is the minor 3rd and B is the 5th.
It's well worth the time and effort to embrace this approach and add a new dimension to your blues improvisation!