According to Justin Sandercoe, a guitar instructor from the Justin Guitar website (see links in Resources), learning the seventh chords and integrating them into your playing is essential for learning blues guitar. The seventh chords are G7, C7, B7, A7, D7, E7, and Fmaj7. All of these chords are similar to the basic open chords with just slight variations. You'll notice that once your fingering is correct and you strum the right strings, that all of these chords sound bluesy, even before you know how to play any blues songs.
Playing blues guitar requires a different strumming pattern than you might be used to playing, but with some practice you'll be strumming the blues in no time. The main difference between a shuffle rhythm and most other strumming patterns is that your strumming hand must pause at the bottom of every strum. The easiest way to do this is to strum down, then pause your hand, then strum up. It's also important to remember that there is only a pause after the down strum, and not after you strum back up.
According to Sandercoe, the Minor Pentatonic Scale is the most common scale used in blues, pop and rock music. This scale is nice because it's easy to play as well as memorize, and it teaches you how to accurately pick individual notes on the guitar. Learn the pattern and make sure you are playing all the right notes, then try to play it faster without sacrificing accuracy. Once you learn this scale, you'll be able to play the notes within the scale and improvise your own leads.
The 12 bar blues "is probably the most used blues chord sequence," says Sandercoe. It's not too difficult and it sounds distinctive once you get it down. The 12 bar blues uses the same shuffle rhythm as most blues songs. However, you only use down strums and only strum two strings at a time instead of four, five or six.
The best way to get the pattern down is to start with your index ringer on the fourth string in the second fret and leave it there. Then strum only the fourth and fifth strings twice, then place your ring finger down in the fourth fret on the fourth string and strum two more times while leaving your index finger still in place. Then lift up your ring finger and strum two more times and continue to repeat this pattern.