Easy to play armchair quarterback, but if each server is storing the complete list, why not just use a queue to push out the change?

    for {
     message = blockWaitingForMessage()
     if message.action == "add" {
       lock()
       blacklist[message.domain] = true
       unlock()
     } 
    }

On startup you'd need to bootstrap a server. Use a database as your source of truth, and wrap an internal-only api to fetch all domains.