Slide guitar playing in blues music had been popular for many years but not many people understand how this guitar playing style originated and the techniques used to produce this fascinating guitar sound. In the early twentieth century many American blues players began their careers playing music on a single string instrument called a diddley bow. This was a kid's toy consisting of a wire stretched between two screws. It seems logical that without access to store bought instruments the emerging adult musicians would develop an instrument based on what they played music on as children. Of course the early blues players did use conventional guitars but a guitar played with a slide made from a knife or a bottle neck more readily complemented the vocal style and blues harp techniques the people used to express their lives in music.
Blues guitar players who took up slide guitar and influenced other musicians to do so were Muddy Waters and Elmore James. Both of these guitarists were driven by the music of Robert Johnson, built on his legacy and further influenced electric blues players like Johnny Winter and Duane Allman. Elmore actually started his musical career on the diddley bow when he was twelve years old. A confirmed individualist, he played a modified acoustic guitar to sound like a solid body electric.
Many students of blues slide guitar think that Earl Hooker is the greatest slide guitar player ever. He sometimes uses wah-wah with his slide playing and often amazed other musicians with his ability to make the slide guitar "sing". But Earl Hooker did not need electronic effects to make his playing great as people who played music with him praise his technical skills. Elmore James' song, "The Sky Is Crying" was covered by modern blues legends Albert King, Eric Clapton, Stevie Ray Vaughn and George Thorogood.
Arguably one of the most high profile slide guitar players is Ry Cooder. He is a skilled guitarist who enjoys injecting his own personality into traditional songs of many genres but he is remembered for his slide guitar on the soundtracks of "Paris Texas" and "Crossroads". For these contributions alone he must be included in any list of blues slide guitar players.
Duane Allman was a great blues player of the late sixties to early seventies probably most widely known for being the "other" guitarist on the Eric Clapton song, "Layla". His mastery of blues music is undisputed and there is a story of the joy he expressed the day he discovered how well a Coricidin bottle could be used as a guitar slide. The story goes he had never played slide guitar before but after that day his slide playing became an indispensable part of The Allman Brothers Band repertoire.
If you want to learn how to play blues slide guitar, you will probably need to learn to play using open tunings, maybe even get yourself a guitar with heavy gauge strings and a high action specifically for open tunings. But to learn slide guitar techniques you can begin with any steel string electric or acoustic guitar using standard tuning. You will need your index finger to damp strings that you do not want heard so experiment with your slide fitted to your middle, ring finger or pinky. You will also want to try out finger picking style playing combined with slide techniques.