The only ones I know of that stay purple are Purple Peruvian, and they are fingerling type potatoes though in great growing conditions can get eight inches long and two inches wide.
There are so many purple (blue) potatoes out now the only way you will find is to search on-line for history of old/vintage type blue potatoes.
The only blue one I know of from that long ago was the one I first saw in a Gurney's catalog fifty years ago.
