Is this soot mold or is it part of the leaf coloration? Soot mold can be rubbed off without difficulty. If this is spreading and is part of the leaf color, is the plant wilting? Is is occurring from the topmost leaves down? Is the plant receiving any chemical or fertilizer treatments on a constant basis?

Top down wilting and blackening could be the result of root rot and overwatering or underwatering. Are the leaves drying out or just wilting and blackening but staying soft?

Chemicals and fertilizers can burn Nepenthes leaves and kill their roots if it leeches into the soil.

Thats all I can think of.

I haven't noticed any drooping at all of the leaves, in fact the plant seems in perfect health - apart from the fact that 3 out of 5 leaves are totally dark brownish color. I haven't used any fertilizers or anything on it. I think it was overwatered. I have my plants sitting in long fibered sphagnum, which was to absorb moisture from the drainage holes, but I think that the moss is wet now from all the misting and the Nepenthes are becoming waterlogged. I will probably repot my lowii, and put all my nepenthes in trays so they don't become waterlogged. An ideal situation would be to have the pots sitting on something like a mesh, so water could drain right through it. Have you heard of anything like this?

Well, I just have mine in a pot with drainage holes and let the water drain out into a tray when I water it, then empty the tray. I suppose you could put mesh over the holes if you need to hold soil media in and let water drain out. If you gave the plant more light, maybe it responded by darkening it's leaves. Mine is producing bronzy leaves occasionally due to high flourescent light levels.