The backgrounds in DD / BD are 2D, so zoomed out they should be the same. With the zooming effect in BD, the backgrounds are not going to look as sharp simply because there is less detail to display on the same size screen. The same thing happens with jpg/png/etc graphics if you zoom in enough.
DD's readme file lists a couple graphics problems that can be solved by checking 'Use AlphaBit'. Apparently is an alternate (I assume slower but more compatible) method to (at least) do color keying and fog.
