There is no significant difference in sweetness between flower-skinned and black-skinned watermelons, which mainly depends on the variety and growing environment. Some varieties of black-skinned watermelon may be sweeter than flower-skinned watermelon, while others are the opposite.

As for why you don't see black-skinned watermelons anymore, it may be because black-skinned watermelons are less common in the market due to their lower yield and difficulty in transportation and storage. In addition, modern people's demand for appearance and taste is getting higher and higher, and the flower skin watermelon is more attractive in appearance compared to the black skin watermelon, so the flower skin watermelon has a higher market share.