The defining sound of most popular music in the 20th century was the sound of blues solo guitar. From acoustic Mississippi delta blues to the wild electrified style of Jimi Hendrix, the blues solo guitar style is what made the sound of blues and rock n' roll. It is one of the simplest guitar solo styles to learn; In fact most blues solo guitar consists of only five notes! These five notes make up the pentatonic minor scale, and an extra note can be added to create what is known as the blues scale.
This image shows the pentatonic minor scale in open position, which is in the key of e minor. This is the most common key for blues solo guitar. While most acoustic guitarists would play E blues using this open position scale, most electric guitarists would play it one octave higher at the twelfth fret, since the electric guitar offers easier access to the higher frets.
This image shows the same scale, but a passing note has been added between the A and the B to create the blues scale. These two scales are pretty much all that are used to play blues solo guitar! You can move these to any fret to play blues solos in different keys. Many guitarists like to think of these scales as shapes on the fretboard, called "boxes." This diagram shows the "box" for an open position pentatonic minor scale.
The scale also forms other "boxes" along the fretboard which can be used, but this is the most common. You can use this to come up with an unlimited amount of licks and phrases to use for playing blues solo guitar or to figure out licks you hear on your favorite recordings. You can move this up and down the fretboard to play in different keys; the most common keys for blues solo guitar are E (open, 12th fret), G (3rd fret), A (5th fret), B (7th fret), and D (10th fret). That's pretty much all you need to start playing blues solo guitar! It's amazing how BB King, Jimi Hendrix, Muddy Waters, Jimmy Page, Eric Clapton, and other great blues and rock guitarists can say so much with just these five or six notes! Start practicing now, and maybe someday you can do the same!