It's hard to know how to write music in a certain style if you haven't listened to much of that style. Said another way, you will compose music like all the music that you have ever listened to.  So, in order to understand how to compose music in the style of the Blues in GarageBand or on a guitar, it's important to have listened to a lot of different Blues songs.  That's the purpose of the iTunes Essentials Blues 101 list. It contains 30 second clips of 75 blues classics that will give you a good feel for where the Blues have come from.  The Wikipedia Blues article is a good overview of what the musical style of "The Blues" is all about.  It is essential reading for any Blues musician. Acclaimed film director Martin Scorsese directed a seven-part film series on The Blues for PBS in 2003 which rivals Ken Burns' documentaries on Jazz, The Civil War, and Baseball.  Some of Scorsese's Blues clips can be found on YouTube.

Berklee School of Music has a number of online music courses, one of which is for playing Blues Guitar.  Instead of paying $1000 to take the course, check out one of their guitar instructors on YouTube as he teaches Essential Blues Guitar Practice Tips.  Finally, to understand how to program GarageBand to play a blues bass line, watch Youtube musician mrslideshow play 12 Bar Blues on Bass Guitar.  It's a great demonstration of how a simple bass line can be expanded into a walking bass line in 12 bar blues.

iTunes Essentials Blues 101

Wikipedia: Blues

PBS: Martin Scorsese presents The Blues

PBS: Understanding the 12-Bar Blues

Scales: The Blues Scale

Berklee School of Music: Essential Blues Guitar Practice Tips

mrslideshow: 12 Bar Blues on Bass