Definition
Blues was the name given to groups if they are a musical form or a music category created in the African American communities in the far south of the United States at the end of the 19th century from spirituals, work songs, field hollers, shouts and chants, and rhymed simple narrative ballads.
The blues category is based on blues form but they possesses other people specific lyrics & use bass lines &instruments.
The term "the blues" refers to the "the blue devils", meaning melancholy and sadness; an early use of the term in this sense is found in
George Colman's one-act farce Blue Devils (1798). Though the use of the phrase in African American music may be older, it has been attested to since 1912, when Hart Wand's "Dallas Blues" became the first copyrighted blues composition. In lyrics the phrase is often used to describe a depressed mood.

History
1899 – Scott Joplin publishes “Maple Leaf Rag”, making ragtime main influence on the Piedmont style of blues.
1912 – The first blues songs, including W.C. Handy’s “Memphis Blues”, are published as sheet music.
1917 – The United States enters World War I. Military and economic mobilization starts the great internal migration of African-Americans.
1920 – Mamie Smith records “Crazy Blues” and it becomes the first blues hit
1925 – Electrical recording technology is introduced and blues music is available for wider audience
1925 – Blind Lemon Jefferson, the dominant blues figure of the late 1920s recorded first song
1929 – The early Delta bluesman Charley Patton recorded first song
1929 – Great Depression in the United States blacks