I was wondering if anyone knows of a blue fleshed potato that turns a beautiful vivid turquoise or robin-egg blue when cooked. It was a total surprise to us because they were purple when raw (with, I think, a bit of white streaking) but I don't remember the skin color- but likely purple too). My brother bought them about 35 years ago at the local farmers market because he thought purple potatoes were funny (before I ever thought of growing my own heirloom potatoes but now I wish I knew what variety it was so I could get some seed potatoes). Any blue or purple potatoes I have found so far either stay purple or turn grey when cooked (although some varieties of the grey ones turn a pretty light lavender the next day after sitting in the fridge overnight) but none are as beautiful as those robin-egg blue ones.

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.