This is not true. Darksend was closed source until RC5 in August/September 2014, when the source code was released. The rest of the Darkcoin source was open, but the Darksend portion was closed until Aug/Sept 2014. I helped write the official press release about Darksend being open-sourced, so I remember it well =)
and I think that worked out well, don't you? I mean, we did have a bunch of complainers/trolls again, saying you can't close source it, but it is after all a company, Dash is, and we should use tactics that protect our intellectual property until it's needed to be open sourced, which is required to be taken seriously in the crypto world for good reason. Still, while working on it, we don't need to let everyone see what we're doing. We can hide until the big reveal just like Apple does.
There are no plans to close source anything. We're just going to build it privately, then open source it as soon as we launch

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Sounds good, so Dash maintains first-mover advantage without the calls of "closed-source!" which at one time were louder than "instamine!".

So true! Over the last year and a half, there have been so many failed trolling campaigns. We've heard:
Darksend isn't anonymous. <-That claim died long ago.
Hard forks will kill your project. <-Ditto
Darksend is closed source. <-Ditto
Masternodes are centralized. <-Ditto
The reference node is centralized. <-Ditto
There needs to be masternode blinding. <-Ditto
Instant transactions aren't possible in crypto. <-Ditto
Now all they have left is the instamine. That implies that they are blown away by the actual technology, to the point where the only thing they have left to complain about is the launch!