Harriet Beecher Stowe's Uncle Tom's Cabin is one of my favorite books: it's deeply humane, while retaining some sense of humor, and it rightly criticizes Southerners and Northerners for their attitudes toward blacks, while simultaneously noting that it is not always easy to find a moral guide. It is also, from our vantage point, a very troubled book, especially from the vantage of someone who wants a truly multicultural basis: "We are all equal," says Harriet Beecher, "after all, aren't we all Christians?" Er, well, not so much, says this atheist Jew.
And for all that some of the black characters are as intelligent and sensitive as any of the white characters (and yes, for all that there is variation in the whites), from our vantage, some of Stowe's characters seem like they'd be at home in any of the pro-slavery literature of the period. After all, it doesn't matter to Stowe if blacks are childlike innocents who love bright colors and to dance--what's important is their Christian souls. But to us, those stereotypes aren't helping anymore. There's a reason that "Uncle Tom" transforms from a saintly do-gooder of the 19th century to the 20th century meaning of collaborator.
Similarly, this anonymous (signed "T.T.") essay-story from the abolitionist Liberator magazine may have a great vision of an America where black and white (and Native Indian, in an aside) are all equal and intermingled. (Oh, Cheerios interracial ad, you're too good for us now.) T.T. writes pretty strongly and with some humor, as when he notes that this future utopia came about after "some bright geniuses made the discovery that black men have rights as well as whites." Also, the frame is an oldie but a goodie, with a powerful end: after thinking about different scales of time, the narrator "visits" the future, finds that it's awesome, and then gets woken up--all of the beautiful sounds of the future turned into the horrible sounds of the slave-trading present.
(That said, I want to address the "well, everyone was racist back then" argument that I usually hear when discussing older science fiction/fantasy works. First, no, not everyone was racist back then, no matter when "then" was. We have so many stories of people crossing color lines and mingling, so there were some people who weren't all that racist.
(Second, the "everyone was racist back then" argument fails to take into account the different types of racism and the different racial arguments. "T.T." here happens to fall into the "races have particular strengths and weaknesses" argument, but he clearly misses the other types of racism that condone slavery--and even skewers them. In other words, "everyone was racist back then" stops the conversation, when we should start the conversation by asking, "what are the different ways people were racist?" As we can see in "A Dream" and Uncle Tom's Cabin, people who thought the races could mingle also felt that there was some innate distinction between them.)