Black Sabbath are almost universally proclaimed as the pioneers of heavy metal. Inarguably they had the biggest impact on the early development of the genre, but it's always interesting to ask what the very first metal song was. Some people will claim it's Black Sabbath (the song, not the band). Others will cite Summertime Blues as played by Blue Cheer in 1968. Personally, I'm somewhat inclined to give the nod to The Beatles' 1968 release of Helter Skelter.
It's all fairly subjective, just like deciding what the first rock'n'roll song was, since it's retroactively labeling points in a gradual evolution. Still, I find the question intriguing, and I'd be interested to hear what any of you consider to be the first metal song.