First let me say I have no experience with WLMM or WMM 6.0.
You could try to change the Desktop Background
Go to Control Panel\All Control Panel Items\Personalization\Window Color and Appearance
and click Advanced Appearance settings.
Click the Desktop color box and pick a color. Apply/OK/Save changes. That may help, the desktop color is used throughout Windows in various places.
Otherwise I think you will need to edit each slide so it has its own "background", enlarge each side of the slide and make it any color. I suspect WLMM is adding the black part to fill its frame and by enlarging a pic to that dimension will make it display right.