Because he wrote about slaves in the slave vernacular (and whites in the vernacular "Pike County" accent) of the place and time, Clemens has been considered pro-slavery and racist. Indeed, I find that unless a writer of the 19th Century was explicit in writing down, in non-fiction, their opposition to racism and slavery they are too often today considered to be both racist and pro-slavery. This quote, for example:

I do not believe I would very cheerfully help a white student who would ask a benevolence of a stranger, but I do not feel so about the other color. We have ground the manhood out of them, & the shame is ours, not theirs, & we should pay for it.

which is from a letter from Clemens to Francis Wayland, December 24, 1885.

I suggest that this article, while about Clemens, might also contain some light on the beliefs of Poe.

You might contrast Poe and Clemens to the Sut Lovingood tales of George Washington Harris.