Who was the first heavy metal group? Some historians claim Led Zeppelin with the release of their
first album released in Jan. 1969. Imo, it was Blue Cheer who released the first album in Jan. 1968.
Their only hit was a remake of Eddie Cochrans Summer Time Blues. They also had the reputation
as the loudest band. Concert goers would often leave the first rows when they played.
I can't really argue with your assertion of Blue Cheer, but I'll argue with anyone who claims Zeppelin as the first heavy metal group. Deep Purple's first album was released in June of 1968, so they precede Zeppelin (who I don't even consider heavy metal in the first place, but just a hard rock band), but more so, the term was coined specifically to describe the music of Deep Purple and Black Sabbath, not Zeppelin.
Younger people will gripe that Deep Purple isn't heavy metal, but that's because they aren't really aware of all the iterations metal has gone through since its inception. I've always considered Deep Purple the original Heavy Metal band since they precede Sabbath's first album by nearly two years, but I may have to concede to you on Blue Cheer. I also freely admit my bias against Zeppelin who I consider musical thieves, and find 90+% their music exceptionally boring.
The other argument, of course, is Steppenwolf, who precede all of those bands (first single released in Oct 1967) and from whose song "Born to Be Wild" comes the term in the first place. However, Steppenwolf always considered themselves to be acid rock, so IMHO that argument is invalid.