Bill, appreciate you trying to get to the bottom of it. But the seller's answer is full of contradictions.
1) He said Biqi is more reddish, yet the picture he shared shows that Biqi is darker than so called "black carbon".
2) in your chat window, he said they have same sweetness level, but size is different. Now he said "black carbon" is sweeter.
3) He said "black carbon" is similar to Dongkui, smaller than Biqi. but it cannot be true, as biqi is much smaller than dongkui already. "black carbon" would be much, much smaller than Dongkui.
I never heard of "black diamond" yangmei. Google search (in Chinese) shows no relevant results. I think the seller just invented a fancy new name and try to sell more trees. Again, appreciate the good things that you are doing, but I have strong feeling against inventing new names for known varieties.
Pretty sure "black carbon" is just Biqi. Both names are legit, and are in most cases interchangeable. I was told that some people call an inferior cultivar "black carbon" (more reddish, less sweet, more prone to fruit dropping), however, but it's not widely accepted as "black carbon".
Biqi is widely considered one of the most productive varieties w/o premature fruit dropping issue. So I'm very surprised to hear that as well.
Definitely I will check with other growers to give folks a clear idea. So, everyone can place order with confidence. Thanks.
"black carbon", "black diamond", "Charcoal", all names for the variety, do not matter. Myself even called it Charcoal at the beginning, but the black diamond truly sounds fancier than "black carbon" or "Charcoal". Just like people introduced the (Myrica) Wax Myrtle tree to China, they called it wax yangmei on purpose.
My personal interpretation from all the resources I read on internet is that Black Diamond or Black Carbon might be a type of variation of BIQI, they are slightly different in color, size, sweetness, etc. Some people in China even say that Biqi could be
black carbon, but definitely black carbon is not Biqi. It's confusing the consumers, right? Just like An Hai, they have soft, hard, and medium flesh/silk, black carbon has big black carbon, and regular black carbon.
For this group order, I am planning to get trees from two vendors, one from Zhejiang province, another one from Fujian province, that's why we can get 4 year old trees in a good price. But so far, I got the answer from both vendors, they said same thing that BIqI and black carbon are different.