CoinJoin is trustless which is orthogonal with centralized or decentralized, it could be implemented several ways (though trustlessness is usually a prerequisite to a decenteralized implementation).
Post 5 in the CoinJoin thread writes in depth about implementing it in a decenteralized way, none of which appears to have been implemented by the darkcoin developers as far as I can tell from what I've heard it seems that they're not even able to understand it. (This is a disappointment to me, since I was trying to describe these ideas clearly so others could understand them.)
More amusingly, what DarkCoin does is highly centralized because the software is closed you can't get more centralized than closed source. What the actual behavior is, is anyone's guess it's impossible to review due to it being closed though "masternodes" does not sound like something decenteralized, it sounds like something that creates a small chokepoint which could be used to deanonymize its users, like a server based CoinJoin but worse since you have to hold a huge pile of coins to run a server.
As I've said before CoinJoin is interesting because it's inherently part of Bitcoin already it just needed better tools (and now there are some, e.g. darkwallet) to make it available to people. It's a privacy improvement over not having it, but it isn't perfect, but it also didn't require any changes to Bitcoin (much less a whole altcoin) to deploy it. In an incompatible system much better is possible as is proposed by ZeroCash and much better is actually _realized_ by Bytecoin (and its forks... Monero, Fantomcoin, etc.), the later are actually working (if immature, due reinventing many wheels) implementations of much stronger privacy, decenteralized in their implementation, all released under a good open source license.
From what I can tell the only purpose DarkCoin serves is to depress me about the state of humanity.
Wow, so I think you need to go back and look at how Darkcoin started. It was my hobby, I wrote X11 in a weekend and launched it. I was thinking I might be able to write the anon tech into the wallet using a variation of Coinjoin, which I started afterward. Darkcoins success was much like Dogecoin, an accident. Please read this: https://darkcointalk.org/threads/the-birth-of-darkcoin.162/
Had I know what was going to happen, I would have obviously taken much more care with the launch and tested everything thoroughly. It's too bad we can't all get past that, it irritates me seeing this constantly everywhere.
If you want to checkout the source for Darksend and see how it works I'm open to that. I think you'd be the best person to look it over before it's opened anyway.