Watch the Full Chords 5-6 online guitar lesson by Matthieu Brandt from Jump Blues
Because jump blues bands can't play up tempo shuffle all night, they often throw in a slow blues for variation. This is one of the very few "minor" grooves we find in the Chicago blues style and it has "Magic Sam" written all over it.
Aside from using a minor chord on the I, it also uses a neat and open sounding voicing on the IV and V chord.
Example 6 uses triad inversions on the top three strings. These are small chords played in a steady rhythm pattern, that are especially useful in a setting with more than 1 guitar or with a keyboard player.