Blues is a traditional African American form of music which has influenced every other style of music since the beginning of the 20th Century. Call and Response is central to the Blues.
Listen to the following example and notice how the guitar plays a response after each vocal phrase. Blues is usually sung with a rougher, more "gutsy" sound than most other styles. This often involves growling sounds made from the throat, along with various slides and bends between notes.
Be careful when singing with these sounds and keep the throat relaxed, otherwise you may risk vocal damage if you do it regularly. If you intend to sing a lot of Blues, it is recommended that you consult a speech pathologist to analyze your technique.
Most song verses divide into eight or sixteen bar sections, but the Blues is usually divided into twelve bar sections (12 Bar Blues). To get a good feel for the Blues style, listen to singers like B.B. King, Bobby Bland, John Lee Hooker, Lightnin’ Hopkins, Bessie Smith and Koko Taylor.