I’ve now tested with the updated version of the G1000 mod in the Community folder, and it works perfectly.
I looked around the settings when I originally had the problem, and didn’t find anything relevant. There is a “Terrain” button on the MCDU::map menu, but that seems to be permanently off. And there is a brightness control for the display backgrounds in the Cessna 172 (and probably in the others), but it never makes the screen completely black (now that I’ve got it working).
So goodness knows why yours isn’t working.