I think your best line of action here would be to send Feedback to Apple for a feature request. As you know, Do Not Disturb can be set to allow phone calls from selected people during specific time periods, they could also include an option to not allow calls from specific numbers during a time period.
As for app recommendations, developers do not have access to the phone number when a call is received, so there would be no app that would be available that could do what you describe. There could possibly be a service similar to a robocall blocker that intercepts calls before they arrive on your phone, but I am not aware of any that let you specify calls to block during a specific time frame.
Product Feedback - Apple