Violet Noodles

Guy making violet noodles in Ciqikou, Chongqing (China) [2017]

Background: Food and the color violet don't go that well together. Usually. This restaurant in Ciqikou, Chongqing (China) serves noodles made from violet dough. It looked like they turn black after cooking but I still did not dare to taste them. I wonder how they produce such color? Maybe with see food?