Index card image background mystery

I have set the index card background, index card image background and media background all to white. Despite this, if I put an image on an index card and want “scale to fit” (which usually results in extra space around the image,) the empty space is always black, not white, and not the color of any tinted card. This happens no matter what kind of image it is.

The background color of the little thumbnail of the image in the inspector does change, but the background of the index card does not.