I realize I'm probably taking your question more seriously than it was intended, but "Dark Shadows" wasn't originally "about vampires" or anything else supernatural. The vampire character of Barnabas Collins was introduced a year into the show's five year run.
When they stand in the sun, they dissolve, so no shadows.
Yeah, I know, that's totally a movie trope and not in real folklore. However, considering the reality of vampires (as you say, none), does it matter?
Don't know about the American version, but in the original Being Human, vampires neither have reflections nor show up on video.
Personally, my favorite vampires are the ones that can be stopped with a handful of sesame seeds.![]()
.I think you've struck on the key point here. Nearly all of the supposed law that exists in popular culture does come from either movies of fictional novels (ie - Dracula)
I don't know that in the original folklore about the returning dead there was an idea that they would not cast a shadow, or that the various cultures that have legends like this have such specific things in common - but I suppose you could go as far as to say people associate hidden and wrong things with night more than day (when it is easier to hide)