Hehe thats easy:
In the WorldEditor, go to "Scenario" > "Map Properties" > "Options". Here you will find several checkboxes, two of them are titled: "Hide minimap in preview screen" and "Hidden areas are partially visible" (Note: This is the exact translation from the german worldeditor, it may be slightly different in the english worldeditor).
Check both to achieve what you wanted. However, if you make a custom loading screen, you may need to uncheck "hide minimmap in preview screen". Note that you can also deactivate the "fog of war" if necessary via triggers. Gl,
klovadis
[Edit:]
eXciTe and Rui, I think you misunderstood what was asked. He didn't want to completely hide the minimap (which I'm interested in too) but he wanted everything to be black at first (like in all campaign maps) and to leave a trace where you are walking.