i’ve tried more than a dozen cultivars of mulbs and nigra’s really are better than the best alba’s out there. By a country mile, at least from my subjective personal taste. Sad that you don’t have it in your country, but i see queensland has started growing mangosteen. If you’ve tried mangosteen, it is the closest thing to nigra mulberries. Mangosteen seems to be sweeter and more sublime while persian mulbs are more like new years’ fireworks in Sydney! 
strange as it sounds, it is almost impossible to grow mangosteen in continental usa, whereas persian mulbs can be grown in many areas here, yet mangosteen is more readily available here(shipped from the tropics, of course). With persian mulbs, you have to grow it for yourself. As fruitnut mentioned the berries are super-fragile and can’t possibly be shipped fresh without special packaging(if at all possible), which will raise the price even more.
whenever available, it is already 20$ per lb at farmers’ markets here. Special handling and shipping costs would not make it feasible to market. Shipping it frozen may be the only option.
