I have grown these carrots along some yellow and purple ones this year. I have found that the “Purple Dragon” ones had an excellent flavour and beautiful colour, but have a tendency to bolt and/or split once a certain size, so they’re better picked young-ish or sown a bit later. In contrast, the “Blanche a Collet Vert” is slower to grow, but unstoppable once it gets going! It doesn’t seem to bolt easily and it can grow very large. The three I have picked today all weigh above between 500 and 600gr!
They used to be grown in many Europeen farms as animal feed as they produce large roots which store very well over winter. Saying that, they are also very tasty in stews!