I am on well with 2 ppm iron. I use softened water to fill but still get about .5 ppm out of softener. So I also run my top up water through a 10" pentec filter housing using a 25 to 1 ppm reducing filter. Cheap and it helps reduce metal in pool and ergo sequestrant use.
I changed my sand last year (after at least 8 years or more before I bought the place) only because the filter is now in a finished pool house and is too hard to deep clean. I figured after recovering a black swamp five years earlier plus incessant sequestrant use, it at least might need the deep clean.
I was surprised my sand was not discolored at all.
The fact that yours is black suggests some kind of oxidization, lack of sequestrant, or something else going on, like gumminess from a different chem reaction in the MetaBgon...or maganese in your water? You may wish to consider trying a different sequestrant, eg Jacks Magic or Metal Magic to see if that makes a difference going forward provided the sand cleans up.
Just a thought.