There should have been a black dwarf in The Hobbit.
This came to me when I noticed the dwarves' noses. Not that it was the first thing about them I noticed; that was their rude assholishness. Their ridiculous noses were the second thing to jump out at me. "Why the fuck did Tolkien make them like that?" I wondered. Certainly, some of them would have a strange nose or hair-beard-braids: that's fair, but jesus-fuck! At least six of them have obvious prosthetics that place me in the theater, wondering why dwarves are the holders of The Seven Rings and the keepers of Middle Earth's greatest diversity of nasal genetic material.
Then I realized that it was because all of the dwarves were white.
Just like I don't harass Star Trek for being "too sci-fi," I can't ding Lord of the Rings for having too much detail. Some of that detail comes from having to differentiate a cast that's either white dudes or fucking monsters. Maybe if there was a dwarf or two that wasn't a short, lilly-white, bearded guy I'd know more than "old one," "fat one," "cute one," "marginally competent one", and "Andy Sandberg one." Disney's Snow White crushes Tolkien with characterized, memorable characters. That's partially because it cheated with the names, but also because it didn't cram eleven of them into a story and assume we'd keep giving fucks after a half-dozen.
As near as I can tell, Tolkien did involve some darker folks in his work. Most of them were distant races aligned with the Sauron, who is LotR's Satan for those of you who've never had to sit through two hours of Peter Jackson films. There were some Hobbits with a rather darker complexion, and Hobbits are the shining bastions of simple folk with good hearts, so that's better, but then better than "allies of low-fantasy Satan" isn't that high of bar. So obviously Tolkien—despite possibly being a bit of a racist himself—put some darker dudes in the source material and they weren't all bad, but none of them really did anything important. I'm not qualified in any way to pass judgment on Tolkien, though with a little research I think he's a pretty great example to use in a larger discussion about the nature of bigotry.
Though it is fair to ask why Peter Jackson didn't put a few non-white folks in Tolkien's work where it wasn't explicitly mentioned. There should be a black hobbit because:
1) Who cares?
2) It matters.
That contradiction is the heart of why minorities and women should be socketed into "classic" stories (and responsibly included in newer works). If a character's non-white, it makes no difference to a good story, especially one not set on Earth in the present day. Revered works should not be so mired in racial identity that they're weakened by accepting non-whites. Holding modern iterations of classic works to the social responsibilities of true representation of non-white, non-males in media doesn't cost the viewing public anything but some of their ignorance and preconceptions.
Meanwhile, everyone--including minorities--values their representation in works. The most damaging message a white-dominated entertainment industry can send isn't that non-white, non-male, non-straight, non-cisgendered folks occupy racial or sexual stereotypes, but that they don't exist at all.
The dominant culture is very comfortable pretending that subcultures don't exist. When they do, they send the message that white, heterosexual men are "normal," and that everyone else who isn't ashamed to be something different is wrong. That's why you have gay pride parades. That's why you have people talking about equal representation of minorities in board rooms, government, and movies; they exist and it's a lie to pretend that they don't. The most egregious lie of omission.
Why didn't Peter Jackson put in black dwarf? Because the fans would have flipped. As I mentioned before, Tolkien's Middle Earth is about more than just putting The Ring into Mordor, killing Smaug, and The Battle of Five Armies. Its fandom is based on the exceptionally dense work done crafting a world, from the broad strokes of mythology, history, and linguistics to the everyday details of foods, games, and local fauna. I would submit that Middle Earth is the details. There are hundreds—if not thousands—of stories about smiting dragons and fighting nebulous sources of satanically-inspired evil. Middle Earth is about lembas bread and elven lyrical talents; abandon that fidelity, and you abandon your fan base.
I'm not exaggerating when I say that every conversation I've witnessed or engaged in about Peter Jackson's movies with someone who's read Tolkien starts with litany of departures from the written canon. Relevance of those points vary, but I've found most to be middling. Even those people generally nod at even the significant changes. After all, Peter Jackson isn't going to wreck the movie like the guy who remade King Kong. Wait. Bad example. What about making Robert E. Howard's tales of Conan into the Schwarznegger Conan films. We're not going to fuck it up like that.
Ultimately it's not too late to add character or that specific breed of moral responsibility into The Hobbit series as Bard or Beorn. He won't though.
[2014, Desolation of Smaug edit: So he added a whole cloth female elf character and put a black person into a crowd. Guess I was wrong about that.]
 No. I've never read Tolkein's work so my knowledge here is not absolute. My knowledge on few issues is. If you're to suggest I limit my blogs to qualified statements, I hope you're prepared to either read obnoxious posts regularly interrupted with qualifications about how I'm not actually a god or become quickly acquainted with the subtle workings of my dick, as that's a subject in which I carry unparalleled knowledge.
 I'm quite sure it was also covered in King Kong.