Looks to me like your sand substrate is a fairly thin layer in most areas of the tank bottom, so even if you pressed some of your root tab ferts down to the bottom of the tank, they're still not very far down, and the root tabs while they disintegrate are leaching their dark color to the sand.
Nothing of a problem if that's the case (which I truly believe it is- cause it's happened to me - with my white PFS), so don't worry about it.
That doesn't explain the rotting plant root ends though - perhaps those plants are just not surviving very well for whatever reason.
My substrate is about 4" in the back and 2" in the front. AS to the plants rotting. It's only the plants that are closest/on top of the tabs. The tabs are all the way to the bottom, there is probably 2.5"-3" above the tab. I can believe their leeching their discoloration to the top. But I'm not getting the plant rotting tho unless it was just too much fert for them and the bottoms are rotting because of it. It's only happening to the plants that are within an inch of the tabs.