Pi-Hole is just a DNS filter. It uses Unbound as the recursive lookup, which gets its info directly from the root servers. So this method is great for getting around provider-level blocks, but if they block a domain at the root level, you're still screwed. But the upshot is that if you know the IP address, you can manually add it to Unbound yourself and continue as if nothing happened. They'd have to forcibly knock the site offline at that point (which can be as simple as just making their provider change the IP address and not update any records).