“Blackball” does not work outside of very small organizations. Most voters will have had very few interactions with any newly proposed core-dev so they will be working with a very limited data set. We would hope that they would trust the champion who proposed the person for core-dev status (and has interacted with them many times) but you can’t guarantee that. You’ll find that there is one person, or a small group of people, that are responsible for all of the blackballs.
I’m OK with requiring a super-majority of the votes, but allowing one person to jam up the works assures that sooner or later you will have one person who jams up the works.