"Hard rock emerged from British groups of the late-1960s, such as Black Sabbath and Led Zeppelin, that mixed the music of early British rock bands with a more hard-edged form of blues rock. Led Zeppelin's eponymous first album, Led Zeppelin I (1969), is a good example of blues rock that represents the beginning of the hard rock genre. The blues origins of the group's album is clear, and a few songs by well-known blues artists are adapted or covered within the album."

~New World Encyclopedia