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.