There is a sub-genre of the blues called Louisiana Blues, with it's music center being New Orleans. Within this subgenre lies a variation called 'Swamp Blues'.
Swamp Blues is a lot more gritty, laidback and for lack of a better term, loose. The beats are a heavier version of the standard blues rhythms or boogie riffs of Chicago and mixing the Cajun sounds the South, taking you to the depths of the swampy, misty Louisiana bayou.
A nice modern example comes from the band Smokehouse. In 1996 the band released an album called 'Cadillac in the Swamp', which features this type powerful sound; a great story along with eerie blues imagery in the video.