Summary
The Classic Major Keystone Lick is useful because it gives you several ways to orient yourself on the fretboard at once. You can anchor it to the B♭ root, see it inside a B♭6 chord shape, connect it to Box 1 minor pentatonic, and understand it as a compact major-pentatonic sound that contains useful target notes for the I, IV, and V chords.
Anchor the Lick
The lick begins on B♭ on the D string and ends on B♭ on the high E string, giving you two clear root anchors. Use those B♭ notes to locate the phrase, then notice how the same area can also be seen through Box 1 minor pentatonic or the major 6 chord shape.
I recommend using minor pentatonic as your main visualization, but major pentatonic Box 1 can also be a useful reference.
Try playing the lick several times while changing your underlying visualization from minor pentatonic, major 6 chord, and -optionally- major pentatonic.
Why This Lick Is So Useful
One small lick gives you clear targets for all three primary blues chords.