Using a generic term such as Dark or Black isnt going to net you many results on Bing. Bing is for refined searches to get more dedicated answers. You would need to add Desktop and/or Wallpaper to your search terms to get a much better result.
I mean that would be like me trying to say search for Blue. Blue what? Blue coins, blue keys, blue skies? Google would give me answers for everything and anything Blue. Bing would only provide me with answer that had Blue in the file name.
That is why Google worked better is because you gave a generic question to search for and it gave all results with the terms provided. Bing was trying to decide what you were looking for. Dark backgrounds of what? Museums, Buildings, murals? There are so many dark backgrounds in the world that can use that term that Bing just gave you the results listed with that in the tag or file name.
If you were to refine the search with adding desktop wallpaper then Bing and Google both would have provided much better Results with Bing offering up some different ones than Google.
Thilly,
Already have that wallpaper.

Official one from the movie release.