It's blackish because of the altitude it's grown in the the regions of Tibet.
The Black Diamond Appleis a rare variety of apple that is grown in the Nyingchi region of Tibet. Despite its name, the Black Diamond Apple is not actually black, but rather a deep shade of purple, almost black in appearance.
The unique color of the Black Diamond Apple is due to the region's high altitude and the intense ultraviolet light that the apples are exposed to. The apples are also grown in a region with a large temperature difference between day and night, which contributes to the development of their unique color.
The Black Diamond Apple is known for its sweet and crisp flavor, and it is considered to be a delicacy in many parts of the world. However, due to the specific growing conditions required to produce the Black Diamond Apple, it is considered to be a rare and expensive fruit.(grok assisted)