Ahh, the thrill of re-bumping and old topic. Since this one never really came to any conclusion, let's explore.
I have noticed that when receiving a new Dionaea specimen (normally of a bare root variety), that they seem fine for the first week, but as soon as the plant starts to really settle in and provide new growth and hope to the grower, that the traps start to blacken. After a short period of time, if this blackened area is left then the rest of the leaf itself continues to blacken. Sometimes it starts out with the teeth of the traps, and turns yellow behind the blackened areas, then finally blackens itself. An interesting note is that this normally effects the trap section first. I have a plant in a very warm and apparent to me now, slightly stagnant terrarium. The plant is now in active growth and putting up a few leaves at a time. The leaves are coming in fine, but once the beginnings of the trap start to fold away from the leaf blade itself, the trap is starting to blacken before even fully forming. The half developed leaves that had existed produced traps that were quite deformed, almost looking like an aphid or fungus gnat larvae attack.
After quite some thought I have a general idea of what causes this, at least in my case.
When you first receive the plant, it's normally in a slight case of shock from being uprooted. This combined with being repotted again during active growth can lead to further shock. If the media isn't sterilized there are micro-organisms, bacteria and fungus normally present to some degree. The plant in shock along with any of these three factors is just begging for one to infect the plant itself.
Another cause that i've noticed that helps the fungal/mold to spread and infect the plant more is high temperatures along with high humidity. Both of these factors can cause mold/fungus to explode into growth. If the plant hasn't fully recovered from the previous shock, it most likely won't be able to fight off any given infection.
Dionaea tend to die off and rot away from what i've experienced when sitting in terrariums with low air flow, high humidity and high temperatures. All these mixed together can aid in the infection in the plant, and cause the blackening of leaves. The dead leaf material then contributes to infection in the rhizome and causes a rot of some sort. All of the factors actually tend to bring further problems not far behind them due to the fact that the plant itself is weakening.
As jimscott has said in numerous posts, along with quite a few others, Dionaea likes its air flow. They are more apt to grow especially well near Sarracenia outside in the free air, soaking up the natural light than with any terrarium grown plant. Notice how naturally not many cp's at all besides Sarracenia grow in area's around Dionaea? Probably a very good hint on the growth of the species.
I'll follow up when I can as i'm actively researching this subject and trying a few things to rectify these situations. It seems to be the most common cause for the loss of Dionaea in my experience.