what would block chain give that git wouldn't? Wouldn't anyone beable to remove a domain or add a domain.

No, you just have a hash of the "official" version of the list. And when the list needs to be updated, the person with the private key publishes a new list again.

The advantage of using the bitcoin blockchain, for example, is that it can't be censored. DMCA trolls now have to go after every single bitcoin company in the world, to stop them from "hosting" the copyrighted work.

Why would they not go after the guy with the private key? Any way he can escape that so could git.

Permanence. The lists would be blockchain addresses that you compile locally into the blocking list.

Perminance is not a great thing for a blacklist. Spamand ads is complicated stuff. There are false positives, and abuse.