Q.My Venus Fly Trap’s Either Turn Black After Feeding Or Reopen A Week Later With The Undigested Fly Still Intact.
The plant is still in its original pot and soil from the store. It’s watered every 3-4 days. It does put out new traps but when eating houseflies the traps go black or reopen prematurely with the fly still intact.

Certified GKH Gardening Expert
They have some specialized care needs. The soil should be moist but not wet. It needs at least 60 percent humidity, and avoid wetting the leaves when you water. Watering twice a week should be enough water, it might be too much. Make sure the soil isn't saturated. Also, use bottled water instead of tap.
The article below suggests growing them in a terrarium. That would provide the humidity and the moist soil. They need help getting food when indoors. Here is more:
https://www.gardeningknowhow.com/houseplants/venus-fly-trap/venus-fly-trap-care.htm
https://www.gardeningknowhow.com/houseplants/venus-fly-trap/venus-flytrap-turning-black.htm
https://www.gardeningknowhow.com/houseplants/venus-fly-trap/venus-flytrap-wont-close.htm
https://www.gardeningknowhow.com/houseplants/venus-fly-trap/repotting-venus-fly-trap.htm