"One can cure oneself of the not un- formation by memorizing this sentence: A not unblack dog was chasing a not unsmall rabbit across a not ungreen field."

It's hilarious, even though I have since decided I don't agree with "curing" the not un- formation.

FWIW, my position is that the difference between strict opposite and negation in English means that there is often a subtlety of meaning accurately expressed with a "not un-". It's essentially the English language equivalent of needing ways to express less than, greater than, less than or equal to, and greater than or equal to. I know Orwell was criticising people who misuse it, but he beats his fists a bit too hard for my liking in that essay. Related, I get annoyed when people aggressively attack "I don't disagree" with "then just say I AGREE!". The phrase "I don't disagree" is useful to convey "I might agree, I don't know whether I agree, I don't have an opinion, I agree with some of it and disagree with small details, any or all of these or more, but I'm certainly not disagreeing with you outright at this moment". In a lot of arguments these opinions fail spectacularly, since for the most part a combination of aggression, loudness, and a veneer of logical reasoning tends to win out over uncertainty and an attempt to consider all cases.