People are often made uncomfortable by elements that reveal racial attitudes in old movies, but those elements can make the movie fascinating. "Dimples", which is set in the 1850s before the Civil War, often makes explicit references to slavery and also reveals 1930s stereotypes. (Also, the movie keeps referring to "the depression," drawing parallels to the '30s.)
The opening legend calls attention, with deliberate irony, to the fact that some young radicals are questioning "that respectable institution of slavery". Then we see Shirley dancing with black and white street orphans, implying that they are equal in their economic straits. Stepin Fetchit has an important but unbilled role as Frank Morgan's servant (who isn't a slave, but isn't getting paid either). Black servants are shown everywhere, especially at Mrs. Drew's house.
Two plot points are important. The central question is whether Mrs. Drew will "buy" Shirley for $5000, and the characters go back and forth on this question. On the night of the debut of the "Uncle Tom's Cabin" play, Mrs. Drew arrests Frank Morgan (in disguise as Uncle Tom). Then while watching Shirley's death scene in the play, where she begs for Uncle Tom to be free, Mrs. Drew "frees Uncle Tom" (letting Morgan go). Shirley converts Mrs. Drew's impulse to "enslave" people.
We see (with historical accuracy) that the play uses white actors in blackface--but in a curious twist, the play closes with a "new entertainment from the South," a minstrel show with the actual black performers (including Fetchit) pretending to be white actors in blackface. These elements make some viewers uncomfortable, but if you can watch critically, it reveals how the movie was attempting at some level to recognize and deal with unpleasant realities of U.S. history and address freedom, equality, and integration in disguise as entertainment. The Hall Johnson Choir appear, and Bill "Bojangles" Robinson choreographed the dances.
