At it's most basic, blues is 3 chords. The I, the 1V and the V (one, four, five). At it's most complex it looks like a bebop jazz standard as it back-cycles through minor and major II V's with delayed resolution. Everything in between is a great lesson - from minor blues, to major blues, to 12 bar blues, to 8 bar blues.
Along the way you'll learn how to use Diminished chords, augmented chords, minor 7ths with flattened 5ths and a whole bunch of music theory you'd never have thought was even in there.
Blues guitar is very rewarding because it's easy to get started and sound good. The great thing about blues is that it keeps giving and never gets old. It really is a musical journey that for some of us, will never end.
If you're in any doubt about how useful Blues is as a guitarist, checkout this article.
One of the best reasons to learn to play the blues is that it has the ability to teach you almost everything you need to know to become a great guitar player.
Of course your guitar has to be in tune (tuned up), but the notes you play with blues - the bad ones that make you wince - are one of the best guitar lessons you can get.
Blues is such a simple form, that it's easy to grasp harmonically - and it's often repetitive. But learn to play the blues on guitar, and you'll also be preparing yourself for smooth progression into Rock and Jazz if that is where you want to go.
Blues training underpins what so many rock guitarists do - For example, Slash or Gary Moore who use blues harmony to shred through hard rock, and Robben Ford's jazz style laced with blues.
Blues teaches you how you can play minor scales over major chords - how you can play notes that are technically out-of-tune yet still sound really good. Blues teaches you to pain and squeal, to laugh and cry on the guitar - how to improvise over chords - how to use modes and triads and double stops in your solos, and so very much more.
Go into any public jam session with some blues skills, and you'll have enough ninja know-how to get stuck in - everyone should learn how to play the blues, it's easy!