I've never used Floramax, or heard anyone else report this problem. The reason I asked if it was black:
A few years back I experimented with dyeing substrate, using homemade concrete stains. Iron for red, manganese for black. The manganese, if highly diluted, can appear pink or purple.
I was just binding these elements to a thin outer layer. Substrate that comes colored from the manufacturer normally has it through and through. But I suspect the same elements are responsible for the colors in all substrates, regardless of whether it's made in a kiln or mined from the earth. I know that the elemental analysis for Flourite Black has much more manganese than the other colors.
It should be bound and stable. Not sure why it would start leaching out to the point of being noticed, if that is what's really happening here. I'm guessing it will stain Purigen, perhaps permanently; but being inorganic, Purigen should have no special affinity for removing it.
Life happens. On semi-hiatus, checking in infrequently.