There are people who will say Budgie, there are people who will say Zepplin, there are those who think Blue Cheer, and many believe the side A of The Velvet underground's "White Light/White Heat" to be the first metal album. All of that is wrong, when you listen to the first Black Sabbath album.
No one came close. And not that early on. Not Yes on their self titled debut, not King Crimson on "In The Court Of The Crimson King", not Atomic Rooster, not Uriah Heap, and not Deep Purple.
Remember; Deep Purple's "Machine Head" wasnt released until 1972. Blue Oyster Cult didnt even release a debut until 1972 (despite being around since 1967 or 68) and didnt really turn heavy metal until "Tyranny And Mutation" (1973). Uriah Heep and Atomic Rooster were keyboard driven and didnt really count towards Heavy Metal (despite Uriah Heep being pretty heavy for rock standards at that time and for Atomic Rooster having some pretty dark lyrical themes).
The fact of the matter is, no one touched Black Sabbath in the realm of heaviness for many years. Not even Van Halen.