Thank you very much Lord Roman <3
I can't recall how many different fixes in different ways I tried that never worked, even made it worse at times and had me reinstall the game in a different folder as a last resort.
Well now my mm9 lets me see clearly during night. Underwater too.
So I'll state the steps that worked for me, thanks to Roman.
right clicked lithtech.exe in mm9's folder
execute with graphic proc >
chose my nvidia card instead of the integrated one
--- it told me I can't do that, proposed me to open the nvidia configuration panel ---
which I did
there I added a new game with lithtech.exe for mm9
set the default graphic card to be used as the nvidia instead of the automatic choice (that used to pick the integrated one I take it, because it was the sole other option)
I'm sorry I can't explain it better for I'm a newbie at this, but I hope this might help some to settle underwater black screen issues.