The idea is that even though you wrote a blog post, maybe sent out formal notices to users, even in some cases put up billboards or did PSAs on TV - some people won't know it applies to them, or may even mistakenly believe it doesn't apply even though it does.
The brownout is an opportunity for those people to notice. "Hey, why doesn't the Foozle work?" "I dunno. Ask on Reddit?" "Huh. Reddit says we need to use tokens, but I'm sure we... oh... did you actually update the Foozle script?" "I thought you were doing that" "D'oh".
Users who can't react immediately when things go bang will get a reprieve - for a while - when it starts working again the next day.
So that's why "blockout" isn't an appropriate description.
There's some degree of art and some science to planning these so that they're disruptive enough that humans notice something went wrong, while not being disruptive enough that they're worse than just a one shot change.