Watch the Slow Blues: Ex. 1a online guitar lesson by Andy Aledort from Kings of Blues & Rock Vol. 6: T-Bone Walker

Aaron Thibeaux "T-Bone" Walker is without question one of the most important blues musicians in history. Born in 1910 in Linden, Texas, T-Bone is acknowledged as the first blues guitarist to use an amplified electric guitar. (Coincidentally, Charlie Christian, the primary progenitor of amplified electric jazz guitar, was a close friend of Walker's and both studied with the same teacher, Chuck Richardson.)
T-Bone grew up in Oak Cliff, Texas (birthplace of two other blues masters, brothers Stevie Ray and Jimmie Vaughan) and learned blues guitar first-hand from playing with Blind Lemon Jefferson.
This example is a slow blues played in the key of G, T-Bone's favorite key for a slow blues. This example represents the first six bars of a standard 12-bar blues progression: one bar of the I (one) chord, G7, followed by one bar of the IV (four) chord, C7, back to two bars of G7; bars 5 and 6 return to the IV, C7.
The riff in bars 1 and 3 is based on the notes of a G7 arpeggio-G B D F-and is transposed to C in bar 2 and played identically. All of T-Bone's improvised lines are based on a combination of the Mixolydian mode (in G: G A B C D E F) and the Blues scale (in G: G Bb C Db D F).
T-Bone always displayed an effortlessness in his playing style, but his lines are actually quite difficult to master. His phrasing is also very free-flowing, as illustrated in bar 4, as he shifts seamlessly from eighth notes to 16ths and 16th-note triplets. In recreating these lines, strive for the same absolutely clean articulation that is an essential part of T-Bone's sound.