In the work "The Prisoner of Food", the reason why the food hunter Acacia turns black is mainly due to the food cell demon Neo in his body.

Neo is an alien creature that once devoured life on countless planets. In the process of fighting against Neo, Acacia turned black because she could not withstand its power. In the process, Acacia's body and mind were eroded by Neo's power, triggering his darkening.

In general, Acacia's blackening was caused by the power of the gourmet cell demon Neo in her body, and was caused by her inability to withstand its power during the fight with Neo.