There's research suggesting it's not this simple, and that Black voting is highly coordinated and communitarian. Already, we have strong indicators that most voting is about personal identification and not about fitting candidates to a personal ideological profile (Achen and Bartels). Combine that with White And Laird's observations about the dynamics of Black voting (heavily identified with the Democratic party, and with an awareness of the influence in the party their bloc voting gets them), and I don't think you can just run a Black Republican and peel off that cohort of Democratic voters.

What's more likely to happen is that the progressive dream of celebrity politicians in places like NYC will run aground of what the party is actually game for, because, as you observe, huge portions of the Democratic coalition are far more conservative than the Take Factory gives them credit for.