According to Delicious Organics:
Purple Potatoes also called Blue Potatoes or Delta Blues truly are naturally purple! This is from the same powerful antioxidant that give blueberries their brilliant color. Purple Peruvian Potatoes were some of the first potatoes harvested. They were saved for Inca kings. Used in Mexican cooking, purple potatoes are gaining popularity in the U.S. They have a naturally creamy flavor and texture and hold their shape well for salads.