Congo is a famous novelty late maincrop with all blue flesh - the colour intensifies with maturity. It is British in origin, possibly from the Scottish Borders, bred sometime before 1895. For decades small (probably diseased) tubers were passed from enthusiast to enthusiast and accurate details of origin have been lost. It was grown commercially recently to make "royal" crisps to be sold exclusively through farm shops belonging to the Queen. They were surprisingly good.