If you ignore blade, which is one of the first real comic book movies you`d be right. I think part of it is the branding doesn`t lead itself into adult content. Even Supes is all kiddy these days.
You are correct, Blade is very violent. Although they messed up the order of his movies. It seemed the first one should have been the last one.
And the new Anime one doesn't do it justice either (some things Americans can do better).
Batman seems to be the one with the most grit and grime to it (of course there are other less exposed DC characters, and lets not forget the other comics under DC as well)
Supes, you can't do anything really violent, unless you have a villain just as strong and violent.
And this movie was great.