Requires Xposed Framework (ROOT) and Xposed Call Blocker
This is only a key! A trial version is available here, please confirm it works before buying this: http://repo.xposed.info/module/com.mohammadag.xposedcallblocker

Xposed Call Blocker is a call blocker implemented on the lowest common userspace level. This means that it's better than any other app at blocking calls without diverting them through the network.

Other advantages include minimal RAM usage, since there's no service continuously running in the background.

Features:
* Block incoming calls on a per-number basis.
* Internal call log that stores which calls were blocked.
* No notifications when a call is blocked, unlike OEM implementations, this allows you to have a completely silent block.
* Block outgoing calls to the numbers specified above.
* Block calls from private numbers.
* Low RAM usage.

Tested on (all running latest stock ROMs)
* Samsung Galaxy Note 3.
* Sony Xperia Z1
* Nexus 5