How dark are you talking? Pitch black? My mod has several pitch dark areas and I did find out that npc's just don't see well in it. The only way I could give them better "sight" was to lighten the area to where you can barely see (but still can see) around you. I chose not to do this as I wanted the "pitch dark" areas. The few Draugr I have will eventually notice you as you will have a torch or a light source. I'll also add that some players hate this. I get complaints. So be prepared to add light sources, torches, "whatever" to help them as well as mention it on your mod page. Which some won't read anyway.