To my ears, the first metal band was Black Sabbath. Everything before it, and in truth a lot of Sabbath's output, was blues rock. This is true of Cream, it's true of Zeppelin, it's true of Steppenwolf and pretty much every other band and song that gets pointed at as being heavy metal. Hendrix, the Who, Iron Butterfly... all blues rock and psychedelia. If we're going to point to stuff like that as being metal, I'd say Pink Floyd's "Interstellar Overdrive" and "Astronomy Dominé" are closer to metal than "Sunshine of Your Love."