Rock 'n' roll, of course, owes its very existence to blues; it is a debt that largely went unacknowledged until the mid-1960's when a number of bands on both sides of the Atlantic began emphasizing the blues inherent in rock, creating the genre of blues-rock.
What the 60's blues-rock bands had largely in common was an adherence to the traditional three chord structure of blues coupled with instrumental improvisation; the epitome of such a band is Cream, although there were many others at the time, some remaining even more faithful to the original blues sound. In England, Alexis Korner and John Mayall led bands that served as a kind of blues finishing schools for young English rockers who would go onto greater fame leading their own bands. In America, many of the West Coast psychedelic bands took their cue from blues; Paul Butterfield, Canned Heat, and Janis Joplin are primary examples; even the Grateful Dead dealt in blues via Ron "Pigpen" McKernan.
The improvasatory jams these bands relied on became as identified with blues-rock as the blues itself; by the late 60's the style began to branch into two different directions. Heavily amplified blues with an emphasis on virtuoso playing evolved into heavy metal, while faster-tempo blues-rock developed into boogie-rock and Southern rock, epitomized by ZZ Top and Lynyrd Skynyrd.
Consequently, by the 70's, blues-rock had essentially morphed into hard rock, and the blues influences began to recede again. Almost all hard-rock of pre-punk 70's has some roots in blues-rock to one degree or another. When punk, new wave, and power pop arrived in the late 70's, displaying little or no blues influence, blues-rock was further marginalized and presumed dead.