Let’s say you’re writing dialogue. And in that dialogue, someone quotes verbatim what someone else said. How do you show that, mechanically?
“We were just sitting around talking, and all of a sudden Josh says, ‘Amy told me she’s leaving me.’ Just like that. No lead-in or anything. Just dropped the bomb on us.”
(I also used the “historical present” in that example. It’s how we talk. Nothing wrong with it in fiction or casual writing, at all.)
See those single quotation marks enclosing Josh’s words? That’s how we know they’re his actual, verbatim speech, related to us by the first speaker (whose name we don’t know).
I see this a lot, also, when people are using scare quotes to let us know that a word or words don’t mean precisely what they normally mean. It’s a problem on a couple of levels. First, those scare quotes need to be examined closely; often there’s a better way to impart the information, one that eliminates the need for them in the first place. Second, if they must be used, and they’re within quoted dialogue, they need to be single quotes. Nested quotes are nested quotes, whether they’re scary ones or reported verbatim speech.
NB: I’m an AmE speaker and editor, and I use CMoS as my style guide. Blog posts reflect this reality.