You know this literally happened this year right? Black voters preferred Biden to Harris and Booker the whole time. That's one of the main reasons they dropped out. They couldn't make any inroads with them.
Harris and Booker didn't stay in long enough to prove this. Neither were ever able to become major competitors either.
Put Harris or Booker in the position of Sanders, in terms of momentum going into SC (say Harris clearly wins NV), and either could challenge Biden. Against Harris, Biden would rely on older white voters to propel him over the top, even if he remains competitive with blacks.