Totally agree that what they did with those old blues songs was completely original and created a new genre. Musicians and other artists don't work in a vacuum, they are inspired and influenced by something. If you look at the history of music, art, architecture; anything creative, you see this. Zeppelin couldn't have done an original composition like "Since I've Been Loving You" without playing with and reworking the older blues standards first. Its all legit and in fact welcome AFAIC. (although I do think its good to give credit to those who influenced you. Not really their fault as that wasn't the industry practice at the time, everything was up for grabs pretty much)