BitcoinDark is just another altcoin. There is no need to sell BTC to buy one of these so called anonymous altcoins if you are seeking privacy as there are plenty of ways of anonymizing your Bitcoin transactions (use a new address every time, use shared coin when you send, use exchanges, use the upcoming darkwallet).
I don't see any of these "anonymous" coins surpassing Bitcoin any time soon.
That is why BitcoinDark is different. It's not 'just another altcoin' BitcoinDark aims to be a platform, providing anonymous support for any cryptocurrency, even Bitcoin itself. Herein lies the elegance in my opinion, instead of creating a coin to try and copy Bitcoin's succes, Bitcoindark aims to enhance rather than compete. If Bitcoindark was 'just another altcoin' I wouldn't dare to post this message in the lions den.