Densuke watermelon

Origin - Japan. Grown only on the northern island of Hokkaido

Days till harvest -

Rind - Almost round. Very dark green.   Flesh - Red, sweet and crisp.   Fruit weight - 11 kg

From Wikipedia:  The Densuke Watermelon has round fruit up to 25 lb (11 kg). The rind is black with no stripes or spots. It is grown only on the island of Hokkaido, Japan, where up to 10,000 watermelons are produced every year. In June 2008, one of the first harvested watermelons was sold at an auction for 650,000 yen (6300 USD), making the most expensive watermelon ever sold. The average selling price is generally around 25,000 yen (250 USD). 

My Notes:   Not sure if these are proprietary seeds. Germination rate is poor I have heard. You can buy the seed on Ebay.