I do not mind it. You have limited knowledge or some one might have fooled you giving a purple termeric saying it is black. You can test it yourself. Uproot it and scratch it. It will show purple color. I am 100% sure. The reason why I say is I have both PT and BT ( Not even a single purple stain in my BT). I need not ask any one. And I have nothing to do with your plants. You posted to Indentify your plants, Keep big mind and heart to accept what is true.