by the way, dear Emperor Jonathan, I don't think one should reduce Berryman's "Mr. Bones" interlocutor in the Dream Songs to "music hall schtick". I know some poets & readers take offense at that particular black-talk imitation, but I think the offense is misplaced. Berryman's alter ego is not a subaltern, but a function of his conscience. Clearly, no one should accept the notion that African-American identity is limited to white Americans' imaginary constructs or fantasies about same. But it is mistaken (and uncharitable) to discount those constructs as merely - inevitably - empty, worthless or malign. They can be ironic, critical representations of cultural landscapes & relations, at certain points in history.
This is obviously a "vexed issue", a controversy, within the general heat of cultural politics, and I don't claim to have the final answer.
I think it could be argued that Stubborn Grew takes the representation - the "schtick", if you will - of submerged (oppressed) conscience, and makes it pivotal to the plot. Some intrepid scholar might want to dig into the sources of this - such as (even more than Berryman) Melville's "counter-Bible of the cosmic castaway", Moby-Dick (cf. Olivia Sachs' weird, hard-to-find Melville study, Game of Creation).