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.
1) Masternodes are just a term. They could be called "decentralized nodes". With 500 nodes on the network, it's hardly "centralized". The number of desktop wallets running will probably be less than the nodes themselves.
2) The argument about centralization and closed source, is invalid. There is no intention for DarkSend to be ...trusted by its users. The open sourcing has been decided and will be done once the code is finished. What's the point of opensourcing it while the specifications are not yet finalized?
3) The coins required for the masternode are in order to prevent bad actors through a cost disincentive that escalates as one tries to accumulate more nodes (less coins in the market, price spikes, accumulation of extra nodes = problematic). DarkSend does not use blind signing, and, if I remember correctly, the reason is that the implementation had DOS issues and the attacker could get away with it. So given that the node knows what it signs, the next alternative was to do multiple darksends through the nodes. That, along with the cost disincentive, would reduce the statistic probability of the bad actor controlling all the nodes of a transaction and hence knowing the money flow. I am not aware of plans to implement blind signing (they could exist, or not - it's up to the dev of darkcoin).
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.
If it was part of Bitcoin, it wouldn't require Dark Wallet, would it?
In an incompatible system much better is possible as is proposed by ZeroCash
How is a trusted solution (due to the accumulator) better?