Perhaps what Mark Twain said at the beginning of the book is the best explanation of the dialects, which were white except for one.
"In this book a number of dialects are used, to wit: the Missouri Negro dialect; the extremest form of the backwoods Southwestern dialect; the "Pike County dialect; and four modified varieties of the last. The shadings have not been done in a hap-hazard fashion, or by guesswork; but painstakingly, and with the trustworthy guidance and support of personal familiarity with these several forms of speech. "I make this explanation for the reason that without it many readers would suppose that all these characters were trying to talk alike and not succeeding." (By Southwestern Twain meant the SW Missouri region of the Ozark Plateau).
I just ran across a quote from Ernest Hemingway: "All modern American literature comes from one book by Mark Twain called Huckleberry Finn."