Harris probably helped turn out black voters enough to flip GA. And for those emphasizing racial relations, picking her was the right start. She has Asian and black heritage which Biden must have thought would help turn out minority voters.
Trump may have gotten the best Republican performance amongst black voters since 1960 while facing Obama's VP who himself was picked by black voters over her in the primary. Descriptive representation doesn't seem what it's cracked up to be...
If he wanted a bit of upside with his weakness with Latinos, picking Beto might have been better option for stopping the bleeding.
...but it's probably still better than picking a guy who pandered incompetently to that demographic at the Democratic debate.
Harris is also less vulnerable to attacks like Sanders or Warren, who are vulnerable to socialism or Pocahontas label. She may not have been the best pick per se, but I can't imagine anyone else being a much better candidate than her.
I'll start: Duckworth, Baldwin, Whitmer, Hilda Solis, probably Abrams when it comes to actually turning out black voters although she'd have her own downsides, to name just a few and limit the pool to female characters according to Biden's original promise.
The narrative around black voters isn't completely true, though. It's very situational and contextual. Some states did have better black #s for the GOP, or like in TX and FL where GOP did better with Hispanics than normal.
However, take PA. Trump somehow did *worse* among black voters (and Latino voters) in Pennsylvania. He got 7% in 2016 and he went down a pt to 6% this year.