Black anger over a primary would quickly end their loyalty to the Democratic Party and doom the party in national elections for as long as they stay at arm's length. After all, without high African American turnout (roughly proportional to their share of the total population) and astronomical African American support (85 percent-plus), Democrats can't win the presidency. They can't win Florida, they can't win Ohio, they can't win Pennsylvania, and they certainly can't win newly competitive states like Virginia and North Carolina. What's more, they'd have a hard time winning the Senate, because without black voters in their camp, Democrats couldn't win seats in any of the mentioned states.
African-Americans wouldn't start voting Republican. But they may well just not vote. And that would kill the Democratic Party.