This is getting off topic. I'll ask how can we tell and then we can get back. This stemmed off of the temporary mountain theory. I still stand by my statement that outside sources would've mentioned such a large mountain appearing out of nowhere and disappearing into thin air. So how do we know they just copied from the Bible?
Given that we have a total of twp sources that mention the sky going black in any fashion, and one of them's only to critique an attribution to a natural cause, meaning we'd have more references to this event than others, it's pretty clear there just aren't too many records of stories from that time. We can tell they just copied the Bible because Thallus is the only one even conceivably close to the right time to have lived through the event, though even being charitable he'd have been very young, and the fact is his explanation conflicts with what, reportedly, he would have observed. It has to be second hand evidence.
But, yeah, even so the original point stands: two surviving sources, one written to rebut. And we don't even have the original manuscripts, just people talking about what they'd once read. We have next to no texts from that far back in time.
For the mountain case you'd need it to be written of, sure. That probably would happen. Then you'd need others to reference those writings: which would happen less, recalling that the sources we have, half of them exist to critique the previous material. Such things wouldn't happen in this case, so one source is down. And equally, this event would be written of less as it's implicit: it'd be far easier to talk about the darkness, say, as the verse is given explicitly, rather than providing a bit of meta and then a reference.
Unfortunately, something not being written of isn't great evidence for or against anything when the source would be that old.