A prerequisite for security is free software. Critical applications like the Silent Circle ones are proprietary, afaict. I have zero trust in the Blackphone and would not purchase one.
1. http://www.theverge.com/2014/2/24/5441642/blackphone-silent-...
They should really have released their code at the same time they released their phone though.
But, knowing nothing about them, when I asked them ``How does your protocol compare to TextSecure's Axolotl?'' the response was ``We have Phil Zimmerman''. So....I'm still a bit put off by them.
Some of their code is already open-sourced here. https://github.com/SilentCircle
Full disclosure: I work for Silent Circle and it's pretty damn secure. It's also open-source: https://github.com/SilentCircle