It’s no overstatement to say that B.B. King influenced countless guitar players, changed the way the instrument was played in important ways. He certainly wasn’t the only one of his generation; in addition to the other two “Three Kings” (Albert and Freddie) of blues, you had (and still have) Buddy Guy, and the late Albert Collins was another contemporary who had a strong voice on the instrument.
But B.B. was probably the most prolific of the bluesmen, and certainly had the longevity, playing well into his ’80s, at a pace that players half his age frequently won’t run.
There’s a pretty decent rundown of King’s approach and solo style here. You may recognize the box as pentatonic box #2 (the major pentatonic), with a passing (blue) note thrown in. This is yet another feature that set B.B. King apart from most blues and rock players — the use of the major pentatonic rather than the usual minor (box #1) pentatonic that everyone uses.
Once your ear figures out the key, it should be relatively simple to pick up the melodic and solo notes. Of course, that’s the easy part with the blues — the real challenge lies in learning to make every note count, to make your phrasing spare, to make the notes sing and swing. That’s the magic, to plug directly into an amp, no effects, no lightning-fast licks, just you and your hands making the guitar sound like another voice, like a woman crying, whether in sorrow or joy. It’s very much a duet, between King’s voice singing a verse or chorus, and then playing a melody or solo on the guitar, as a response from the other vocalist.
Again, the note choices are critical, but not in the sense of having to memorize a pattern or box or scale. If you hit a note your ear perceives as “wrong,” just remember that the “right” note is next to it, one fret up or down. Then your wrong note becomes a passing tone. Even a “wrong” note played with conviction is better than a “right” note played half-assed. Keep it simple. Think of your tone — the amp, the guitar, the amount of pressure you use in your picking and fretting hands — as your speaking voice. Think about what you’re trying to say on your end of the conversation.
Watch the way B.B.’s left hand shakes as he squeezes every last drop out of a note, that’s vibrato. Hendrix had it, Stevie Ray Vaughan had it, Jeff Beck and Billy Gibbons and Joe Bonamassa have it. Great vocal bending control and powerful vibrato are among the strongest tools a lead guitarist can have. B.B. King helped forge that path for the rest of us, a true musical pioneer.