@Bill88 I'm sorry you lost your mantid from such a problem.
Overfeeding is not a issue in nymphs, and even in adult mantids, does not cause death directly. Overfeeding can lead to death due to prolapse (seen a few times online where their insides are pushed out their abdomen (anus)), or more commonly a fall where they are easily punctured on habitat decor.
Many mantid individuals will turn a dark color (nearly black) after death especially noticeable on the abdomen and head. That is just a form of their limited decomposition and not necessary black death - decomposition does of course have a bad smell itself. If the habitat is kept warm or has higher humidity levels the darker coloring is more noticeable as well.
Black death is a bacteria issue, and the signs of it are the mantid vomiting black tar-like substance that has a strong odor. They usually have it stuck to their mouth and palps as well. Such vomit should is visible in the habitat itself (and requires cleaning to remove), usually on the walls. Normal vomit has a reddish color, easily seen through with light, and does not have much odor. If you do not see any vomit, then the so-called black death is not likely.
A substance excreted from the abdomen after death is normal and is again likely decomposition (some individuals do it much more readily than others, and depends on the amount of food in their
mid/hindgut).
If ventilation was a issue though it could very well be a bacteria issue, as the lack of air flow allows bacteria to build to high levels and often mold is a problem.
The bloated abdomen could be a build-up of gas, or unlikely fluid. If it happened while the mantid was alive it suggests the bloat problem was present before feeding, likely a bacteria issue, as it would take sometime to become a problem. At least if the bloat is significantly larger than the feeder(s) it ate, and not normal digestion which I imagine it is not. If it occurred after death, in mammals it is common with decomposition, but I have not seen or heard of it in mantid decomposition.
All things considered it does sound like the mantid developed a bacteria problem; although, not the black death version. If you have any other mantids I would advise you to clean their habitats first, especially the one the mantid died in. Then increase habitat air ventilation, and offer any mantids 100% pure honey. The honey is antibacterial and will help any mantids affected with bacteria.
The best honey is from a bee keeper (farmers market, organic stores, etc) to ensure it is real honey, as most stores do not sell real honey even if it is labelled as such (store versions are over purified with no pollen, and often have additives). Put the honey on a toothpick or bamboo skewer and offer it to mantids near their mouths. If a mantid does not seem interested, smearing a bit on it's raptorial forearms will give it a taste as they will clean it off giving it the benefit (and the mantid will likely devour it next time it is offered).
For cleaning replace all substrate, boil or replace sticks/branches, and wash the container (and anything that is permanently attached) with soap and hot water, and rinse well. A more effective cleaning method is solution is made with bleach. Dilute bleach with water, some use a few drops of bleach per cup of water; however, even one teaspoon of bleach to a cup of water is only a 2% solution mix (
50 teaspoons are in one cup). A alternative to bleach is vinegar, and dilute with water too.