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.

Because this way you only read the data in once and don't need to make any changes to it which allows you to pick different data structures.

May be.

I read the article twice but had the exact same reaction on both occasions that somehow we used a "big" bat to kill a small ant when some easier solutions would have sufficed.

Though, it's looks very likely that I might be missing something really minuscule.