A lot depends on what kind of wood they're in - both the base color and how quickly it absorbs. Like if it goes into really old untreated pine it'll be practically grey within a day, but in cedar (which is red to start with and pretty porous) it stays kind of a mahogany-ish red.
I'm ok with that explanation, sounds reasonable. Not really the kind of research I care to do, surely "blood splatter staining on unfinished wood" is a search parameter likely to get you on a watch list.
Damn why didn't I look up!?
Those planks do look like they may be cedar.