he wants the minimap to be black while he's playing, not in the gameroom before starting the game...
In your map MPQ-archive there is the war3Mapmap.blp (you can open it with any MPQ-Program)
You have to safe this *blp as *tga (Do this with a war3imageExtractor) and you must draw it completly black... Then you can safe it as *blp again and import it per MPQ-Program into your map-archive again...
FINISHED!
Then your minimap is black btw... you can also edit it into a nice picture That looks good ingame