If you needed evidence beyond my presence in this forum and that of my neighbor Heimhenge that people are listening to your show in cities where the local public radio outlets don't carry you, look no further than your first caller this week.
Mary Jo West (who asked about "dark as Egypt") was already a very prominent TV news anchor in Phoenix at the time I moved here over thirty years ago, and is well-remembered to this day.
It seems odd that "light dawns on Marblehead" is used when someone *finally* catches on to what e3verybody already knows, when the town of Marblehead MA is one of the first places in the US to experience dawn. As Yakov Smirnoff used to say, "Wotta langiuage!" There's also a Marblehead, Ohio on the north coast, in Ottawa County. (Note that Ottawa County is Put-In-NJay and Port Clinton, but not Ottawa. Ohio which is in Putnam County. Go figure.) Since the expression is strictly Bostonian, no danger of Marblehead, Ohio being involved.
Oh yeah, I recall Mary Jo West from when I moved here in the 80s. I hadn't heard "dark as Egypt" before but immediately got the meaning ... and I'm no bible scholar. Surprised that West, coming from a Bible study family, didn't pick up on that.
I've heard that Marblelhead explanation before, but it had slipped outa my vocabulary. Rarely use it, but occasionally hear/read it.
Likewise "Apple Core Baltimore!". I grew up in Wisconsin and that's one of those dumb childhood games we used to play. Another was when you saw an empty pack of Lucky Strikes on the ground. First one to stomp on it got to declare "Luck Strike!" and do a knuckle punch on another's shoulder. Was particularly "fun" around construction sites, where the ground was littered with worker trash of all kinds.