This problem also appeared when I did my own bean sprout experiment. At that time, the online answers were "the surrounding environment is too high" or "too much watering", but it was not. The bean sprout device with black bean sprouts is placed in an air-conditioned room, which has no shading, so it is absolutely cool and breathable, and there is water leakage at the bottom of the device, which basically does not exist. Later, I found it when I wrote a course paper and looked up the information. ! ! When bean sprouts germinate in winter, experienced bean sprouts farmers will raise the watering temperature and give them enough heat to make them grow normally. You can refer to the article "Problems and Countermeasures in Bean Sprout Production" written by Cao. Attached is a picture of your bean sprouts turning black for your reference.