Undertones are so low that for the most part they're inaudible. You don't have to go down many octaves before they become say quarter notes or eighth notes. A bit more and we're talking about one bar and four bars. Consider that two notes can be many octaves away from each other, and if they're perfectly in tune they will still harmonize. This is also true for undertones. The 12 bars of the blues can be thought of as one octave that is several octaves below audible sound. So the first bar and the sixth bar are a tri-tone apart. Notes played on the 1st, 5th, 9th bars will make an augmented triad. So this reminds us that where we play in the form causes harmonies to be created many octaves below our range of hearing. Even though our ears don't pick up these consonances and dissonances, our intuition may and we certainly perceive them as sounding good. The bigger point that I'm getting at is that form follows the same laws of consonance and dissonance as audible notes do. This is the reason that harmonious proportion is important to achieve when structuring your solos. There is a tri-tone relationship between the first bar of your solo and the mid-point. Between the first bar and the point two-thirds into your solo there is a perfect fifth relationship. If you play every other bar you create a whole-tone scale. You can set up effects that are like chords in a certain way by becoming aware of these nodes/harmonics. In a blues it is just a bit easier to deal with these ratios because the form of the blues perfectly reflects out equal-temperament system.