Just in from Justin Turrell:
Dark Wallet is a technology which pairs bitcoin spends of similar amounts amongst their customers, and co-mingles their coins together before sending them to the payee, in order to make the payments harder to trace on the bitcoin blockchain. In this respect it's something like a coin laundry, except that the function is made part of the act of making a bitcoin payment, instead of a separate optional step.
SilentVault's approach is superior to this in several ways:
1. Our payments do not take place on the blockchain at all. The only footprints are when blocks of coins are transferred into or out of SBC (Silent Bitcoin). It is impossible to know (from bitcoin's perspective) what the velocity of SBC is internal to our network. Therefore the longer a particular coin sits in our reserve, the greater the likelihood that it has been spent many times before being withdrawn by the user who redeems their SBC voucher for SBC and claims that coin.
2. Our system supports more than just bitcoin. We also support litecoin and our own currency, Silent Silver (SSV). Additional issuers are contemplated for gold, USD, EUR, and CAD. We could also adopt additional alt-coins, including those from Ethereum, Mastercoin, Ripple, etc. All of these different types of assets co-exist in the same wallet.
3. Payments do not require another user making another payment at about the same time, in order to provide enhanced privacy for both users. A single user can make a private, off-chain payment which only the payee will ever see.
4. Because vouchers are destroyed in every transaction, and replaced with new vouchers bearing fresh serial numbers, it is not possible to track the flow of money, for any of the asset types. This is digital cash without a public ledger.