Hi and welcome to the forum! Hope you find it friendly and helpful. I took the liberty of moving your question to the Fruit Growing forum, where a lot more people will see it who might know the answer.
I don't grow watermelon because I don't have the room for them and when I tried the critters ate every one, while they were still little.
I tried looking it up and all I got was this:
"A black watermelon fetched $6,100 at a Japanese auction on Friday, making it one of the most expensive melons ever sold. The 17-pound black-skinned "Densuke" watermelon drew the unusually high price for its rare color." https://www.slashfood.com/2008/06/14/black-watermelon-sells-for-6-100/
Maybe you accidentally got a rare black watermelon seed mixed in with the regular ones?
(Just kidding, don't try auctioning it off!)