BlackPhone is scam and snake-oil. Cloud "privacy" with remote control. Need privacy? Use burners.
What's fair to say is that you're implicitly trusting Silent Circle to be both competent and benign. But then running pretty much any modern computing device requires you to trust various groups of people in that regard, so this isn't that different really.
The only alternative would be coding the thing 100% yourself.
The only question is of who you choose to trust and how informed your decision is.