The blues borrowed, scholars contend, the theological architecture of the spirituals, with devils and hellhounds ever on the singer's evasive trail. So it is that the blues also introduced a key American cultural trope, the voice that sings only to proclaim it has vacated its spot; the blues singer often professed to have been someplace and then to have gone. This facet of the blues was indulged largely in the Mississippi River Delta and Texas Piedmont regions, where blues singers played guitars, sang, and either tapped vigorous rhythm with their feet or utilized various rhythmic instruments—the triangle, for example—to keep time.
A key element in the evolution of the blues was the creation of what came to be called blue notes, flatted thirds, fifths, and sevenths that recurred throughout a song's progression. The growth of the blue note is vital to the development of all American popular music, for it became central to much jazz in the 1920s, 1930s, and later. Further, blue notes ended up in Broadway stage shows, among the first vehicles to draw in music fans with performance and recordings, rather than with the plainly detailed sheet music that had, in the early twentieth century, enforced a kind of musical correctness to which the blues would never measure up.
In the twentieth century's first great North American migration, Delta and Piedmont natives, blues musicians among them, traveled in massive numbers upriver and went urban. From the Mississippi River, singers disembarked at St. Louis and took root or headed west to Kansas City or, more often, east for Chicago and New York. It was in the latter city that the first of at least two waves of "urban blues" took what was largely a folk music and translated it into a staple of popular culture. In the 1920s, female singers especially found a vibrant market just as recordings were becoming the backbone of the United States's economy in music. Bessie Smith is merely the most famous of these women, many of whom recorded what were then considered licentiously bawdy songs built from double entendres, songs that seemed related largely to love, both carnal and spiritual.
Meanwhile, the rural blues took root, as well, with folklorists beginning in the 1930s to document the musicians as part of the New Deal's cultural records projects. Robert Johnson (1911–1938), one of the music's most acclaimed progenitors, was, in this way, identified and recorded before dying at the age of 27. Others, however, took the guitar and vocals modus operandi to large cities and began adding two critical elements: drums and amplification. By the 1940s the blues of the Delta and Piedmont had transmogrified into rhythm and blues, only to be followed by another wave of urban blues that relied anew on the rhyme scheme, harmonic properties, and rhythmic formulations of the music's early creators.
Once the presence of electric guitars, drum kits, and amplification became de rigueur for urban blues musicians, their style of the music became preeminent in the national imagination. So popular was the electric, throbbing blues of John Lee Hooker and myriad others that rock and roll began to dovetail with the music, much to the long-term consternation of fans. The "British Invasion" in rock and pop during the 1960s drew much of its inspiration from the blues, a cultural phenomenon reminiscent of the Fisk Jubilee Singers' large-scale popularity abroad. The difference in the 1960s was a much more refined popular-music economy, where youth culture was quickly becoming engrossed in rock and roll at the same time that the struggle for African American civil rights took cultural center stage. Rock and roll won the day, even if the blues remained popular for such artists as B. B. King, John Lee Hooker, Bobby " Blue" Bland, Etta James, Koko Taylor, and others throughout the 1970s.
Scholars have tended to look at the blues as fodder for memoirs and documentation or, on occasion, as elemental to African American literary history. In whatever guise one investigates the music, there are countless matters of historical and cultural interest—from the interchange of musical economies as the blues helped nurture rock as a burgeoning youth movement to the development of rural blues in relation to spirituals and secular music of the nineteenth century to literary and poetic tropes developed by blues singers and picked up later by poets, novelists, and others.
Encyclopedia of American Studies, ed. Simon J. Bronner (Baltimore: Johns Hopkins University Press, 2015), s.v. "Blues" (by Andrew W. Bartlett), http://eas-ref.press.jhu.edu/view?aid=202 (accessed April 1, 2015).