Nice idea, but falls short

The receiver of the messages has to know what password was used to encrypt each of your messages (if you dont know their password, you cant read their messages). Assuming youre texting this person, theres a good chance that password will be sent in a text, making it available for anyone who really wants to read your messages.
You have to individually copy and paste (which is impossible to tell when the message has been pasted into the app, no paste button) each time your want to send or receive an encrypted message. There are several other apps out there that offer an alternative to using an standard SMS or MMS that dont require constant copy and paste (many of them are free!). It would be much simpler to send the messages over the internet directly through the app so that you are not jumping between apps all the time.

mbaim7 about Black SMS - Protected Texts