It's plain and simple, the only reason Black people vote Democratic is because under a two-party, winner-take-all system, they don't have any choice. They always fear that any vote for alternatives, while closer to their principles, doesn't mean anything in a practical material sense, and will possibly lead to victories by the racists in the Republican party. Trust me, Black people don't just view Republicans as "conservatives", they view them as racists, white supremacists, and potential KKK members, which is why they are so feared, and why the scare tactics of the Democrats work so well.
If we had proportional representation at the national level and a radical political party that would push things like a national living wage, reparations, universal health care, free universal education, etc., the Democratic party would be ground into dust.
The issue of proportional representation in some form is the key to breaking the grip of the Democrats on Black folks and the working class in general, and one of the reasons why the Democrats will probably fight the hardest against it. The Democrats are probably more scared than the Republicans of what would happen if PR ever went national in this country.
Art