The big mark appears to be in the middle of the plank. You should do a dry wipe of the smudges and take it to the county for analysis. Rule out pathogen.
Zagat, I do believe you've jinxed me! Here's a photo of the biggest smudge today.
Did your grandson pay you a visit?
Could be a scuff from the sole of his shoe.
How long in inches is that largest smudge in the kitchen?
Zagat, I do believe you've jinxed me!
oh, btw do you have maids cleaning in your house? It may be something that they're using
Zagat, I do believe you've jinxed me! Here's a photo of the biggest smudge today.
It wipes off easily with windex. Mold would be a powdery black. This is a greasy black that will smear and can be tracked to other areas. Besides, it appears suddenly.
The space between the floorboards looks much worse than it is.
The longest dimension may be around 2".