I had 3 watermelons on my plant that were as big as your thumbnail. Now there are 2 gone and one is black as the ace of spades. What happened?
My Problem Is on Watermelons
Certified GKH Gardening Expert
It is either a pollination problem or a calcium deficiency. I would guess it is likely a pollination problem. This article will help with that:
If you see immature fruit falling off or shriveling, you may need to hand pollinate. This article will help with that: https://www.gardeningknowhow.com/edible/fruits/melons/hand-pollinating-melons.htm