In the late 1960s, the term heavy metal was used interchangeably with hard rock, but gradually began to be used to describe music played with even more volume and intensity. While hard rock maintained a bluesy rock and roll identity, including some swing in the back beat and riffs that tended to outline chord progressions in their hooks, heavy metal's riffs often functioned as stand-alone melodies and had no swing in them. Heavy metal took on "darker" characteristics after Black Sabbath's breakthrough at the beginning of the 1970s. In the 1980s it developed a number of subgenres, often termed extreme metal, some of which were influenced by hardcore punk, and which further differentiated the two styles. Despite this differentiation, hard rock and heavy metal have existed side by side, with bands frequently standing on the boundary of, or crossing between, the genres.
What band had a 1960s break through with an emphasis on darker themes?