Im just going to copy and paste an old comment from /u/CitizenTed
There's some misconceptions being bandied about here. I'd like to set the record straight. Fact is: Black Sabbath really was the birth of heavy metal. Forget Steppenwolf, The Kinks, The Who, and all that. Black Sabbath was an entirely different and shockingly dark, heavy sound that NO ONE else was doing at the time. NO ONE.
Led Zeppelin's first record was out and found a huge audience, but that record was dirty, heavy blues - a well-worn road. It was hard and melodic, but it was also light and sexual in tone. Black Sabbath was a whole 'nother thing.
Imagine this: it's February 1970. The biggest radio hits are "Raindrops Keep Fallin' On My Head" and the Jackson 5 "I Want You Back". OK?
Now, listen to this
You will probably discern a meaningful difference in tone, temper, and imagery in this music. It was the most diabolical, explosive, and macabre sound ever put to record. Y'all young folks are so used to stuff like Metallica and whatnot as a constant thread in the tapestry of modern music. But before Black Sabbath, there was NOTHING.
Black Sabbath was much more than distorted guitars. It was an all-encompassing, mind-bending world of doom and power. It was the template for heavy metal, and every heavy metal act since then has tipped their hats to Black Sabbath and every single one of them knows it.