I always got the impression that it was more of an colloquialism as opposed to an official label.
That’s what it should’ve been, but it always was something of an official label. There’ve been self-described dark Jedi, and most sources capitalize the “D” in “dark”.
I prefer “fallen Jedi” as a term for Jedi who went bad.
That sounds a little better, but doesn’t that communicate more-or-less the same idea though? Plus I don’t see why a darksider would destcribe him-/herself as being “fallen”. “Dark” is at least a term such a character might use with some pride, and it is descriptevely accurate, albeit kinda odd.
Maybe it’s a slang that became commonplace, post Palpatine’s purge of Jedi history? (It’s a decent head-canon at the very least.)