Why do my venus fly trap leaves rot a few days after starting to digest a fly?
Answers:
Probably the reason that your trap turned black is that it caught a fly that was too big for it to digest. The general rule of thumb is prey should be no bigger than 1/4 the size of the trap. After a trap has caught an average of 3 to 5 meals, it will die back naturally. Check out the link I've provided. On this site you will find care sheets for the Venus Flytrap as well as other interesting carnivorous plants. On the yahoo groups link, there are great people that will be more than happy to answer any questions you may have about VFTs or other CPs.
Other answers:
too attract more flies from the smell
too attract more flies from the smell
Try bicarbonate of soda.
overwatering
Dead leaves and heads are a part of this plant's life cycle, and do not necessarily mean that the plant is unhealthy.
Always keep dead leaves and heads cut off to prevent fungal infections.
indigestion
Harry is right! Venus Fly Traps are native to my state, and can survive UNDERWATER for periods of time if nescessary! They are originally found in bogs here in the Carolina's. So you almost cannot overwater one... but the dead leaves are part of the cycle of the fly trap. Keep the dead stems cut off as close to the base of your flytrap as possible without disturbing the root system, and don't feed your plant flys yourself! Let it catch them on it's own (if you're not) as they may only catch 3 or 4 flys in a lifetime on their own! They survive very well on the nutrients from the water they sit in. ( you DO have yours resting in a saucer of distilled water, don't you?) Just give it plenty of bright but not hot sun, and plenty of distilled water to sit in and drink from, and let it catch flys on it's own. Mine is so big I finally divided it, and two of the traps had grown so big they caught each other! They won't let go, either! Good luck and happy gardening! :)