I've worked with a book called The Roving Editor a few times now for various projects. It was published in 1859 by James Redpath, a reporter for the New York Tribune and radical abolitionist. The book was essentially the compilation of a series of newspaper columns he had secretly written and sent north while traveling the South via various covers and temporary newspaper jobs. His overall goal was to obtain secret interviews with slaves wherever he could while also reporting on overall conditions of slavery and the South. I find it to be a highly significant travel account because, as you might imagine, most firsthand accounts by slaves were collected after the war whereas these interviews were conducted before slavery's official end (publication in 1859, travels/interviews in 1854 and 1857).

Redpath is about as radical as it gets when it comes to his abolitionism and advocacy of rights for African Americans, yet often his transcriptions of his conversations with slaves show highly different language styles depending on who's speaking. Slaves certainly speak with a different style than Redpath himself or even white Southerners. So how do the nuances of spoken languages get onto the page?

Redpath wasn't able to record or copy out every syllable of his interviews, so he used stenography (a standardized style of shorthand, the specific kind escapes me right now) to compile his notes in real time. In other words, his notes did not include anything denoting the vernacular styles, accents, etc of the people he interviewed, except perhaps word choice. This means, so far as I can tell, that he must have recreated these dialects when turning his notes into a newspaper narrative after the fact. Redpath took active control of the formation of his interviewees language and clearly altered it according to the race and circumstances of the person involved. What we read in The Roving Editor is not literal transcriptions of African American vernacular but rather Redpath's mimicry of them, subject to his own perceptions, memory, and interpretation of it.

Did Redpath purposefully distort his interviews? I doubt it, as it would have run counter to his overall persuasive goals regarding slavery as well as his stated beliefs about the intelligence and humanity of African Americans. But Redpath was, like everyone else, subject to his own biases and perceptions of what he encountered. Abolitionists were certainly not exempt from racialized ideas and beliefs.

The idea of "accuracy" of language in topics like this is tricky. A person like Redpath or Mark Twain (since you mentioned Huck Finn) are certainly going to have these biases and interpretations of African American language and culture, and it will affect their portrayals. But writers for the public also need to craft their images in ways which their audiences (who themselves have all this "baggage") will accept as legitimate. Perhaps everyone involved was creating lies they could agree upon.

History is sometimes viewed the same way, as lies agreed upon. It is at times difficult and/or problematic to think of history as containing some completely accurate record of, say, African American vernacular rather than something with slightly blurred edges, made problematic by the incomplete and fallible record we have at our disposal.