Album cover for John Lee Hooker's House of the Blues (1960).
In 1940 the Second Great Migration had begun, eventually relocating around 1.5 million African Americans from the south by the end of the decade. For the blues, the consequences were momentous. Cities such as Kansas City, Chicago, and Oakland, centers for jazz and swing, introduced arriving musicians to new, innovative styles including large-scale orchestral arrangements and (with the exception of those from Louisiana) brass-heavy jazz ensembles. Most importantly, it was in these cities where amplification was taking off. The outbreak of World War II brought better employment opportunities and, consequently, a more discerning and demanding black music consumer base. Thus, the conditions were ripe for the breakout of southern-born electric guitar virtuosos such as Aaron "T-Bone" Walker and Charles Brown (from Texas) and Muddy Waters and John Lee Hooker (from the Mississippi Delta). Stylistic differences, however, still remained among the now-electrified musicians: for example, Hooker retained much of his original "country" manner, while Walker created an entirely new sound, "expanding [his guitar's] tonal capacities so that it could phrase lines like a saxophone" (Morthland).
Morthland, John. "Jump Starter." Texas Monthly Apr. 1991: 68.