[dragonwriter]

> That's an odd convoluted definition that I've never heard of before

It's a “definition” in the sense that it is a description semantically equivalent to what the phrase “dark side of the moon” has always referred to; it is not, however, a description of the etymology of the term, which derives instead from the long established use of “dark” to mean mysterious, hidden from knowledge, obscure, occult, etc.