Jazz is originated in US, called "degenerate music" and "musical masturbation". It is imported from to Louisiana by the French. At the very beginning, the players were composing when playing. Though it is ad-hoc, it is totally driven by the player's mood and emotion. It is almost a free form to express themselves. Rawness is the style.
At the same time, most of the African American were working in the fields as slaves. Their life was tough. Though they tried to enjoy the work, you can still feel the sadness through their music (Blues).