NaCl is open source. The client software itself is considered to be open sourced.
Regardless, if you think that way you can also not trust the TextSecure binary you've downloaded frome the Play Store nor the Android phone you're using.
I guess you've trust someone or start building your own hardware, mobile phone OS and instant messenger and make everyone else use it as well, otherwise there's no one to communicate with. However, those people will be in the same dilemma:can they trust you?
