(CNN) Even after first debates, Joe Biden still holds a 20-point lead with black voters

[Kind of Blue]

11. South Carolina before the debate June 20th reported in South Carolina

https://www.cbsnews.com/news/in-south-carolina-biden-is-democrats-top-choice/

From your link, "Biden leads among black voters in the Palmetto State with 41%." That's a big drop in South Carolina in less than a month.

Nationally back around June 20th, again using CNN as the source, An average of our CNN polling in late April and late May shows Biden lapping the field among black voters with about 50%. Harris, Sanders and Warren were all between 5% and 10%. Biden's 40 point lead among black voters was more than double his about 15 point lead among white voters.
https://www.cnn.com/2019/06/22/politics/joe-biden-2020-black-voters-poll/index.html

Now, and finishing off a sentence left out in one of your paragraphs, Harris is still about 20 points behind Biden among black voters overall. In an average of four live interview national polls (ABC News/Washington Post, CNN/SSRS, NBC News/Wall Street Journal and Quinnipiac University) taken after the debate, Biden holds 39% of the black vote. Harris is in second place with 20%, which is only 5 points ahead of Sanders' 15%.

Statistics is not my strong point but surely even I can see that he has a comfortable but shrinking lead. Those are the trends that I look for and we're just getting started.