Now this is a wonderful little find!
It's a very cool video of the late B.B. King performing “How Blue Can You Get” at Farm Aid on September 22, 1985.
What makes this performance so special is what happens about three and a half minutes in. King breaks a string on his guitar and changes it himself—onstage—while he continues performing the song.
All the strings are still clearly attached to the guitar at the three-minute mark. The video then cuts to a shot of the horn players, and when it returns to B.B., the highest string is dangling from his headstock.
At around 3:12 you can see a roadie in a blue cap running behind the band; a few seconds later he hands a replacement string to King. After nonchalantly removing the broken string, King winds the new one as he sings up a storm.
This is pure showmanship and professionalism in action. It's one more reason the guitarist remains so dearly missed.