Watch the One of BB's online guitar lesson by Jeff McErlain from Solo Factory: Chicago Blues
This lick is so synonymous with BB King that pentatonic position it is derived from is often called BB's box. Although we are using the lick on the V chord, it can be used anywhere in the blues. The lick uses mixture of the major and minor pentatonic scales. I choose to look at it as an A major pentatonic scale (ABC#EF#) that replaces the 3rd (C#) with the 4th (D). By using the D instead of the C# all the notes work over each of the chords. The C# is a very poor choice on the D7 (IV7) chord as it is the natural 7th. By saying poor choice I really mean it will sound terrible! Really spend some time with this lick on all the chords of the blues and you will see how truly important it is.