The reason why loofah turns black when cooked is because loofah contains high sugar and melon seeds contain melanin, which easily turns black when exposed to high temperatures.

Solution: Put the cut loofah in a basin before cooking, sprinkle some salt on the loofah, and stir it with chopsticks. After about five minutes, water will come out of the loofah, and it will not cook again. It will turn black and look particularly nice green. After the water boils, put down the loofah and don't cover the pot.