Blackberries, blueberries, pomegranates, and other dark berries contain a dark pigmentation that can cause staining on teeth. Tiny organic particles in berries can penetrate the pores in your tooth enamel and remain attached. If your teeth have a higher porosity, this will lead to more persistent stains. That's why your teeth can turn purple and may continue to have a purplish hue after eating blueberries.