Hey Harry,
As for the how the "Black Pearl" tastes, it has a very subtle sweetness. It is not a super sweet fruit, but definitely not on par with a Water Chestnut. Unfortunately, I have not tried any other Cultivars to compare it against.
I would say that if a water chestnut is a "1" on a scale of 1-10 for Sweetness, then the "Black Pearl" is a "6".
Hope this is helpful.
Jacob