Most call blocking apps will reject/hang up the calls from blacklisted numbers. But the calling party comes to know that there call was rejected. Is there any way or any app which can really "block" the call (like turning off network connection for few seconds) so that the calling party feels that the phone is either switched off or not reachable.
This is a network function and no app will be able to perform the behaviour you're looking for.
At best, you can create a custom "silent" ringtone for a known number. Else, if you'd prefer pressing the volume buttons will usually mute the ringing while giving the dialler the perception that the call remains unanswered.
That would be more of carrier dependent and network related, plus, the programmatic API is not well documented hence a lot of third party call blocking apps, inevitably, do send a reject/hangup upon receiving a blacklisted number.
What the OP is asking for, is actually, going down deeper into the nuts and bolts of telephony layer which is blocked off due to:
If that was available and documented, there would be a lot of call-blocking apps on the Play store, that can do the call-screening in the proper way that would be expected of.
As an afterthought to this answer, a vital part should be noted - Google blocked the
I'm not sure about manufacturer skins like Samsung's TouchWiz, but AOSP has the feature you're looking for.
In Google's contacts app, for any contact entry, there is an option to send 'All Calls to Voicemail'. When someone rings you with this enabled, it will ring for 5 to 15 seconds and, of course, go straight to voicemail.
Alternatively, you may want to look at this app. I haven't tested it, so I cannot confirm that the behaviour between the two is identical.