Privacy and Open Source can be the same thing -- the idea being that if we know how something was built, we can make sure there aren't any backdoors / "hidden" features that we don't know about. Most if not all of the open source operating systems still include a binary baseband blob, although work is progressing on a few (the best one I saw only worked on really, really old phones).

One of the main criticisms of the Blackphone was that A) PrivacyOS (I think it was called) was seemingly black box (no pun intended) -- they said they would open up the code soon, but we haven't seen anything yet B) It still would use a binary baseband. The illusion of security is sometimes worse than no security, and open source code helps with removing those barriers (Although it doesn't neccesarily -- just look at OpenSSL).