Can the black vote can make a difference in a primary? I don't know. I think you are right about mayor Pete & blacks.
However, about the general election? I've been wrestling with this for years and I still don't see how any one minority group can sway a national election.
We need to look at individual states where they can make a difference.
Some states are already RAT without them.
Obama didn't get elected because of the black vote.
Interesting dissection of the black vote here:
https://www.nbcnews.com/politics/2020-election/journey-power-history-black-voters-1976-2020-n1029581
Also interesting here, but based on exit polls.
https://www.mic.com/articles/159402/here-s-a-break-down-of-how-african-americans-voted-in-the-2016-election
as of 2018 the 10 states with the highest black population as a percentage: (one drop of black blood is considered black) how many of these went to Pres. Trump in 2016?
D.C. 50.7% African American
Mississippi 37.3% African American
Louisiana 32.4% African American
Georgia 31.4% African American
Maryland 30.1% African American
South Carolina 28.5% African American
Alabama 26.4% African American
North Carolina 21.6% African American
Delaware 21.0% African American
Virginia 19.9% African American and the vote by race: Does Pres. Trump win without the 8% of the black vote he did get? (I'm pretty sure he does)
2016 %GROUP CLINTON TRUMP
RACE White 70 37 57
African-American 12 89 8
Hispanic 11 66 28
Asian 4 65 27
Other 3 56 36
I think Mayor Pete also turns off enough whites.