what caused this new small dark hole on my hardwood floor recently?
It's likely a knot that came loose.  The circular grain around it sure makes it look that way.  The wood can turn blackish like this if water gets in there and sits which could easily have happened with a small hole like that.



How a Pin Knot is formed. Broken branch with encased dead twig, when the branch split under snow load, the twig slid through the surrounding wood, surviving intact. Same thing happens on tree trunks when small branches die and don't break off.