> Two blocked domains with the same hash isn't a problem, they both need to be blocked.

This is a problem if the second one is actually a major useful domain. If HN and adserver have the same hash, you have a problem. This could be described as a false positive for HN.

> Where hash collisions matter is false positives, e.g. if hash(google.com) = hash(adserver.com)

This sounds like the same issue I describe above where you need to use Google but block adserver. Otherwise it's only a problem if you implemented allow lists, right? This is when you explicitly allow Google and implicitly allow adserver along with it because of the hash collision.