BlackPhone is simply a stock version of Android with the best settings and with secure apps installed. It's really nothing different than what you can do with your own phone. If you have the appropriate skill/knowledge, you can configure your own Android phone to be just like BlackPhone. It also comes with subscriptions to SilentCircle, a VPN service, and a cloud storage service, which may be cheaper as a bundle with installed separately on the phone.
BlackPhone does fork Android with their "PrivateOS", but such a fork is of limited utility. Google innovates faster than a company like BlackPhone can keep up, including security innovations. A true fork would quickly become out of date with Google's own patches, and hence be insecure. BlackPhone is still new, so I don't know how they plan on dealing with this. Continually forking the latest version of Android seems the most logical plan, if not convincing Android to accept their changes.
The upshot is this: if you don't know anything about Android security, and you want the most secure Android phone, then get a BlackPhone. You'll likely be more secure than trying to do everything yourself. Your calls over SilentCircle will likely be secure. You'll likely not get pwned at WiFi hotspots. But that doesn't mean you'll be perfectly secure -- Android is a long way away from that.