The Blues style evolved in the Southern states of the USA in the late 19th century. Its origins lie with spirituals and work songs as sung by enslaved African-Americans. The first form of Blues that is identified with the name is 'Delta Blues' - a style that emered from the Mississipa delta. Since then, Blues has evolved into many different strands.
Blues harmony has two essential areas: dominant harmony which is derived from the Major Scale; and Minor harmony. During the course of the 20th century, Jazz musicians applied chord substitution to basic Blues harmony to create many harmonic permutations.
The Blues is often characterised by a triplet feel called a Shuffle.