If the trap was fed something that it wouldn’t normally catch in the wild, this could cause the trap to turn black. For example, beef, chicken, or pork would all most likely cause a trap to turn black. Insects with hard shells, such as beetles and roly-polys can also cause a trap to turn black during digestion. Also, if the trap is getting old or the weather is cool or cold, it might turn black before it completely digests whatever was trapped.
Another reason that the trap might turn black is if the prey that it caught didn’t completely fit inside the trap. If this is the case, the trap will try to seal unsuccessfully. Then when it releases its digestive enzymes, the trap will start turning black because the enzymes seep out onto the trap surface.
However, sometimes leaves on Venus flytraps just turn black for seemingly no reason. Don’t be too concerned with this unless all of the traps on your flytrap are turning black. Then you might want to try to figure out what’s causing a general decline in the health of your Venus flytrap.