Even if you've only heard a handful of B.B. King songs, keep in mind that King played his notes on his guitar "Lucille" sparingly, utilizing a signature vibrato technique to make his notes and melodic phrases ... sing. King apparently wasn't interested in strumming chords; he let his backing band do that. King's solos and lead licks, however, played to the chord and therefore these mini-melodies often had a chordal quality to them.
To put this in perspective, the basic rock/blues style of guitar playing will only play to the key, not the chord. If the chord progression is in E, major or minor, then the guitarist would likely improvise using the E blues scale, regardless of the chord changes underneath (as long as the overall key is E):
B.B. King, as many have observed, played within the B.B. Box, which is a certain zone on the thin strings of the guitar, up or down the neck, depending on the key of the song. I am being wildly inaccurate and overly simplistic in my analysis, but King basically fretted four or five notes -- and bent a string to get to other notes he wanted to play.
A typical 12-bar blues chord progression in E starts with the E dominant 7th chord, E7, for four bars.
It is perfectly acceptable to play the E blues scale over the E7 chord. It is also valid -- and harmonically logical -- to play the E Mixolydian mode over the E7 chord. Some guitar experts say that B.B. King played the major pentatonic or a fragment of the major diatonic scale over the first chord of the 12-bar blues. For simplicity's sake, let's say he went for a bit of the Mixolydian mode. In our example, it's the E Mixolydian mode:
The blue-colored notes represent the B.B. Box, or at least an approximation for the situation. Only five of those six notes should be fretted; it would behoove all of us to bend the B string a full step from the F-sharp note to the G-sharp note, and give it some B.B. King-style vibrato for good measure.
The next chord for E blues is the A7 chord, for two bars.
At this point, B.B. King tended shift the B.B. Box into something more bluesier, namely, the blues scale. In our example, it's the E blues scale:
The blue text indicates the state of the B.B. Box for this scale. Instead of bending to the G-sharp note, which gives the scale a major key flavor, bend to the G note instead, to give the scale a minor key flavor. (Even though it's not in the above chart, you can bend one half-step from the F-sharp note to the G note, to make it easier for your fingers.) There is a chromatic run from the A note to the B-flat note to the B note. The B-flat note is what is usually known as the diminished fifth, or the blue note in the context of the blues scale. In other words, when the band is playing A7, either play the E blues scale and/or an E almost-Mixolydian mode with a G instead of the G-sharp ... if that makes any sense.
The chord progression returns to E7 for two more bars, so return to the Mixolydian mode, major key notes and all.
The next chord in E blues is B7, for one bar.
Play the E Mixolydian mode or the E blues scale. Mix it up. Just play.
The progression returns to A7 for one bar. Play the E blues scale again.
The closing chord and turnaround for the 12-bar blues in E tend to be E7 for two bars, including perhaps some A7-Bb7-B7 riffage (approximately) at the turnaround to the next 12-bar sequence. Play either E Mixolydian and/or E blues -- just make it sound good.
For reference, let's look at the B.B. Box "zone" for E Mixolydian and E blues side-by-side:
When he played the B.B. Box, B.B. King did so much with so little. He didn't have to shred 16th and 32nd notes to make a statement; he either bent the string to the minor 3rd or to the major 3rd, depending on the chord. He shook a note around at the end of a phrase. And Lucille sang the blues, every time.
My humble tribute to this legendary guitarist is but a simplistic analysis of just a small part of B.B. King's mastery of music. B.B. King sang and played the blues, and his very existence and influence will forever be a part of time itself, never to be erased, and never to be forgotten ... and so, the thrill remains.
R.I.P. B.B. King.