How should I setup my minimap in Starcraft 2?
Normal background, it just helps you put things in perspective of the map. I don't think I've seen any top level players use a black background.
Most players prefer setting enemy units to red on the minimap, this helps to alleviate your issue with seeing them against the backdrop. This sets ally units to yellow and your own units to green no matter what your actual colors are. I believe the command to do so is Shift+Tab.