I don't really see how throwing blood and gore at something makes it dark (if anything they made Origins look childish). The darkness of a setting comes not from the number of gallons of blood you see on screen, nor even the colour pallette used, it comes from the characters, and the stories, it comes from the way people and things act and react. You can easily have very dark settings where everything looks bright and cheerful. The heart of darkness is candy coated (imaginary cookie for those that get the reference).
Overall, I'd say Thedas is a pretty dark setting. The world is not a nice place to live in. We see opression and prejudice. We see war and it's brutality. We see a world on the brink of destruction.
But yet, despite that, I consider Origins to not actually be that dark. Why? The story. Ultimately, depsite the darkness of the world it takes place in, the main plot of DA:O is remarkably optimistic. Yes, bad things can and do happenover the course of the game, but in the end, our intrepid hero can unite the people of Ferelden and lead his merry band to save the day from the forces of darkness. Throw in the ritual and this can happen pretty much without cost. There's no negative consequences to your actions. There's no sacrifices. Sure, you can play in a way that involves taking darker decisions, but in a truly dark story, you would have to do so - or at least, if you didn't, there would be major consequences. DA:O's story is a heroic fairytale, that happens to be set in a crapsack world. Of course, that isn't neccesarily a bad thing, as indeed, it's a well told and enjoyable story. There's a place for idealism, for escapism. Not everything has to be grimdark. And in many ways, putting such an optimistic story in such a harsh world makes its message even greater - that even among such horrors, there is light.
DA2, by comparison, IMO, is a much darker story (and before someone jumps in here, I'm simply talking about the underlying plot and not taking notice of any flaws in its implemetation). In the end, Hawke can't save the day, Kirkwall will collapse into conflict no matter what he does. There's no idealistic happy endings, there's no triumph of heroism over darkness. Sometimes all you can do is give your best, only to find that your best simply isn't good enough to save everyone. You can help, you can make things less bad than they might have been, but in the end, you're just a man, and there's a limit on what one man can do. It's a much darker, more realistic outcome than Origins.