1. Call blocking apps, like in Google Play, need extension for developers that Apple is not currently supplying.
2. A true call blocker actually "rejects" calls (pickup & hangup etc) from the blacklisted numbers, and does not even allow callers to leave a message.
3. Apple's is also a call blocker but it simply directs calls from blocked numbers to voice mail, although it won't ring and you won't realize that callers actually left voicemails until you check it.
4. The current approach by Apple for call blocking cannot block "unknown" or "private" callers etc.
5. 3rd party apps like Hiya and Truecaller etc have huge lists/data base of spam callers (millions?) and those apps help screening unwanted callers to some extent, but it's almost useless IMO.
6. I used to jailbreak and used an app called iBlacklist, which is a true call blocker, like those you find in Android world.
7. Some carriers offer call blocking service at cost to block private or unknown numbers etc.
8. But in the end, current approach by Apple is good enough, and I am no longer obsessed with call blocking.