Neo is an alien creature that once devoured the lives of countless planets, and Acacia was blacked out because of its unbearable power in the process of confronting it. In this process, acacia's body and mind were eroded by Neo's strength, which led to his blackening.
Generally speaking, the blackening of Sophora japonica stems from the power of Neo, a gourmet cell demon in its body, and is caused by its unbearable power in the struggle with Neo.