I'm going with Amistad here instead of Django Unchained, because while the slave revolt aspect is interesting in its appeal to moral intuitions, Amistad more directly incorporates elements of coercive commerce.
The Atlantic slave trade, for those of you who remember your high school history, stood on three legs: the Africa-to-Caribbean chattel run, where most of the horror occurred and where most of the cinema is set; the Caribbean-to-Roanoke sugar run; and the Roanoke-to-Africa rum run. Other triangles, such as with Boston or Lisbon as vertices operated at various times. The details are not particularly salient for the moral questions hinted at by the movie.
We need neither the wisdom of sages nor the patience of ages to know that the forced transportation of captives for bonded labor is immoral. The East-West leg of the triangle was an abomination. Does this also imply that the other legs were as well? One ship, one skipper, three loads of cargo. Is the sugar necessarily soaked in the blood of the men and women pitched overboard upon sight of a British Naval vessel?
I think most people would say "yes" for both consequential and deontological reasons. But how about secondary markets? How about knock-on firms? Does a pastry chef fold the innocent tears of orphaned children into his brioche every time he buys a pound of sugar from his wholesaler? Is that dram of rum spiced with shackle iron? How far does the contagion of immorality spread?
It's not an idle question either, confined to the distant mists of an uncivil past. Violent drug cartels (for example) claim quite a bit of shadow GDP. How many degrees of separation from their money laundering operations do you need to be to claim moral cleanliness? Does the fact that gangsters might by proxy use foreign deposit institutions render these so-called "tax havens" dirty?
And how about across time? The US rail network was subsidized by the non-euvoluntary labor of Chinese immigrants who were treated horridly during and after their work was finished. Is part of the locavore movement underpinned by an aversion to frequenting these artifacts of oppression? I haven't heard this argument forwarded with any seriousness before, but absence of evidence is not evidence of absence and all that, you know.
I admit to being a little ambivalent. I won't intentionally buy stolen goods, but if you put a few more transactions between me and the original thief, and don't make it an express point to tell me about it, I think I would probably be okay with it. Similarly, if I buy a hamburger made from cows that were fed with hay harvested by farmers who drove tractors assembled by slave labor, I'd be distraught at the idea that there exists slave labor, but my outrage would be entirely disconnected from my meal. Heck, the country I live in used involuntary labor in conduct of war and I don't lose sleep over that. Unless I make the specific effort to reflect on it, of course.
I pretty freely admit that this is my own moral calculus. I also admit that your mileage may vary. Because there's lots of moral elbow room here, I also find myself extremely leery of legislation that tries to hem in the scope of the market based on these sorts of moral contagion threats. If there's a problem with a product or the way it's made, it seems a lot more reasonable to expand the choice set available to the laborers. It's hard for me to imagine a society getting systematically richer by curtailing the extent of its markets. Peace, easy taxes, a tolerable administration of justice. That's what's called for, not embargoes, not boycotts.
Same goes for the current furor over Russia's treatment of homosexuals. You want to help Russian gays? Petition your government to offer them asylum in your home country. Boycotting vodka or the Olympics helps them how, exactly?