It's just shorthand. There's a "blind" condition. Use it to represent what happens when you're in the dark. But, just to keep the rabid rules lawyers in check, we're making it clear that you aren't literally blinded by being in the dark because, well, that'd be stupid.
Personally, I see that as being for the "rabid" (not the word I'd use) simulationists, not the rules-lawyers. Then again, I am perfectly okay with accepting the idea of a
prone gelatinous cube, so I may not be defining "rules lawyer" in quite the same way you do.
Edit:
Also, the thing I find funny about this errata is that a true "natural language" reading would seem to indicate that being surrounded by a cloud of unnatural darkness
doesn't prevent you from seeing anything outside it, so it acts more like a column of total invisibility. Unless being inside it makes everything
outside it also "obscured by it," in which case it would seem to be literally verbiage without a point. Other than, as mentioned, the "appeasement" meaning. To clarify my "simulationist" thing above, the appeasement would seem to be, "Okay, okay, we get it--being trapped in thick fog doesn't mean your eyes
don't work, it means your eyes
have nothing to work with. We get it. Here's a phrasing that doesn't imply your eyes stop working."
I mean, unless there's some kind of extra-special feature or something that only works on people with the Blinded condition? I don't know enough about 5e to respond to that, other than to say that I've never heard of such a feature, nor anything that would exploit it that *couldn't* exploit it with the errata'd text anyway.