Q: Every fall this hard black gunk appears in one exact spot on my hardwood floors. It appears to be coming up through the crack, and the only thing that removes it is my steam mop or a sharp blade. I’m not concerned about HOW to remove it, but more so about what it this black gunk and how do I prevent it from coming back?
A: I’m not certain but it almost sounds like you had your floor stained dark and some of the stain and finish seeped between the boards where, devoid of fresh air it has stayed soft and every once in a while squeezes up to the surface. However, this phenomena called poly balls generally occurs in spring and summer when humidity is higher and you can get some expansion of the boards.
As a side note, I would not recommend a steam mop be used on a hardwood floor.