If we grant, for the purpose of discussion, that the substance Amanda and Raffaele were tracking around---in the hallway and in her bedroom---was not blood, you do have to wonder how it got on the soles of their feet. Whatever it was, we know that they didn't paint it onto their feet with a paintbrush. Instead, DUH, they must have stepped in it. And yet no source of that "mystery substance" was detected with Luminol...such as a pool of turnip juice under Amanda's desk. Okay, so Amanda or Raffaele, wiped up the spill after they'd both stepped in it while barefoot, using, say, a rag or a paper towel. But in that case surely there would be a shadow of the "mystery substance" left behind, which would show up under Luminol as a large smudge or smear. But, to the best of my knowledge, not even a shadow of pooled "mystery substance" was detected.
I know it's been suggested that the "mystery substance' is none other than water found in the bathroom shower, possibly contaminated with rust or cleaning agents. But that don't make no sense. If true, wouldn't there also be Luminol-revealed bare footprints in the bathroom too? None found. And besides, has anyone ever said that Raffaele ever showered at the cottage? Also, if the water is contaminated--- and since Amanda showered daily---wouldn't there be many more of her Luminol-revealed prints left from previous days?
But speaking of smudges, it so happens that Barbie Nadeau does mention "smudges"---smudges of blood---visible on the floor of Meredith's bedroom. ("Angel Face," page 168 and again on page 169.) When you conjoin this with the undisputed fact that there is also a bare footprint in blood, on the bathmat, a few feet away from Amanda's and Raffaele's bare footprints in "mystery substance," the simplest explanation is that the "mystery substance" in none other than blood. And if blood, there would be no mystery as to how they stepped in it, any more than how Rudy stepped in it.
"The simplest answers are always the right answers." ---Steve Moore (Elapsed time of videotape=35:24, KIRO FM)