The power of a dark fog need not be arbitrarily amplified by placing conditions upon it. Fog is created through the following conditions:
* Draw water on a large scale
* Bring the ambient temperature to the dew point

Fog usually "burns off" when the ambient temperature rises (e.g. when the sun comes out), so to create a long-lasting fog, you'll want to control the ambient sunlight. The cheapest way, mana-wise, is to create a sort of cloaking spell that bends sunlight away from your fog zone. Note that optical cloaking will improve the darkness of your fog even when the sun is gone, so in that respect, a more complex spell is more powerful. It's not the limitations placed on your spell that strengthen it, per se, but the depths to which you create it. The obvious downsides, of course, are mana cost and casting time.

Does that help?