The TW modding Youtuber GudeaTW told me that it is possible to have the pitch black fog of war on the campaign map like in Medieval 2 (it was introduced there in order to cover the Americas before their discovery).
I did the following:
1) Deleted these files:
"C:\Program Files (x86)\Steam\steamapps\common\Total War ROME REMASTERED\Contents\Resources\Data\data\world\maps\base\map.rwm"
"C:\Program Files (x86)\Steam\steamapps\common\Total War ROME REMASTERED\Contents\Resources\Data\data\world\maps\base\map_heights.hgt"
2) Copied "C:\Program Files (x86)\Steam\steamapps\common\Total War ROME REMASTERED\Contents\Resources\Data\data\world\maps\base\map_features.tga" and named it "map_fog.tga"
3) Painted everything black (0,0,0) in map_fog.tga
4) Started the game (normal Rome, not BI or Alexander): map.rwm does not get recreated, but the campaign for some reason still works? Unfortunately no changes to the fog of war were made.
What am I doing wrong?