terraces
In your enhanced image, there appear to be thin terraces to the crater with dark coatings. On my stretch of the "black" I see similar variations of very-dark-grey to charcoal to midnight-black. These are the sort of morphology we see in many Martian craters, so I suspect there is a VERY dark dust or basalt filling the crater. But why no wind streaks of it downwind, or the light surrounds into it.
It reminds me of the black feature that was thought to be a crater lake some years ago, but proved to be dark dust filling the smooth base of a "normal' crater.
I'll do some more work here, but beware of 'enhancing" jpeg's. You're often enhancing artefacts...