If it's a common place word from what I understand you can only protect the industry you do business in and there can't be prior use. Marvel and DC are the old dogs and probably were the first to use the term superhero in the comic book industry. When I think first superhero I think Superman and that is DC all the way. If there were any predecessors they are most likely no longer around to contest.
I'm not sure "Superhero" is a common place word though. I mean, it is NOW, but prior to it's introduction into popular culture I would think it probably didn't exist as a compound word, so that would only go to strengthen DC's case.
Not a trademark lawyer, just my
