Hello,
What are you feeding your VFT?
If they are large meals, then the trap will be exhausted by the one meal, which is normal; but I don't think that it would turn black that fast. I don't know how much your plant is benefiting from only being closed a couple of days before turning black. Somewhere I read that if the insects have pooped or are heavily bacteria laden to begin with, for whatever reason, then the added bacteria, etc. will kill the trap. I heard also that if there is an appendage (leg, wing, etc.) sticking out of the trap, then the trap may not closing adequately to make a good seal to keep bacteria from growing on the trap. I'm new here, so I hate to offer advice this soon, but I've done exhausting reading both in books and in this forum and am only repeating what I heard.
Hope I helped some.