Fix for black screen in D3D (Paper Mario: The Thousand Year Door and probably more games)!
Change to a compatible D3D Blend State right before the ovr_EndFrame() draw call.
Some games (e.g. Paper Mario) do not use a "Blend State" that is compatible with the Oculus Rift's SDK.  Changing to settings used by Mario Kart for GC fixes this issue.
To Do: Only use this when needed?  Which "Blend State" setting breaks the ovr_EndFrame() command?  Is it an SDK bug or expected behavior?