This is how the call blocking API on iOS works. The blocker app can only provide a static, pre-set list of numbers to block to the OS, and that's it. It has no access to call history, awareness of calls being received/made, etc. The OS handles all the blocking, referring to the blacklist the app provided earlier, and provides no feedback to the app itself about this.
Of course, this means that call blocking apps have less features than on Android. For example, apps can't dynamically look up a number when a call is revived and make an on-the-fly decision. This is in keeping with iOS' philosophy of "privacy/security over features", vs. Android's "everything is completely open to developers, for better or worse".