Really? It seems that it is a very easy thing to do.
Obviously you need to grant permission to the app to monitor every inbound call, but I am sure that it could be an open source project that is not using that data for anything apart from the stated purpose.
I'd pay for the same on Android
FDroid has a very good call blocker. It integrates with everything so you don't even remember it's there.
But I didn't try to block calls with no disclosed id. Those disappeared from my view a while ago, so either it does block them by default or something changed on the part of the callers.
yeah, come to think of it, I don't get a lot of blocked ID calls anymore, either. But I do get multiple spam calls a day, which is what I took OP's complaint to mean.
