I wonder if it would be possible to make a whitelist blocker instead of just blacklist? I mean, if a certain domain is in the whitelisted list, you show only re

I wonder if it would be possible to make a whitelist blocker instead of just blacklist? I mean, if a certain domain is in the whitelisted list, you show only responses from whitelisted subdomains. If not, you fall back to blacklist. By possible I do not mean technically possible, but feasible in the resources required maintaining the list as well as good enough user experience.