Clearly there are upsides: some people are more comfortable with the suggested terminology.

Why is "blocklist" or "denylist" harder to learn for beginners than "blacklist"? If anything, the approved terminology seems easier to learn, if you ask me.

It sounds like your argument is not so much about beginners learning for the first time, but experienced people needing to learn a different word for a concept they already know. I'm fairly sure I can learn what "blocklist" means with minimal effort, but perhaps that's a difficult word for some people.