...interested in to the black Fluorite sand and would like to try that out some time
I would definitely go with the black fluorite sand. It has the same grain size as Seachem Onyx Sand but the color is better than SOS and unlike SOS it won't buffer the water to a higher PH(IME, using SOS did not negatively effect my fish or plants despite any buffering, perhaps because I used a dusting of peat underneath it).
Here is what I found. I am not saying that you will find the same thing. I set up two tanks. One at home and one at work. In one I used a cap of Tahitian Moon sand over leonardite, mulm, and peat moss and in the other I used a cap of Seachem Onyx Sand over leonardite, mulm, and peat moss. The plants and lighting in both tank was the same, and excel was used as a carbon source. Both tanks got a pinch of phosphates, nitrates, potassium, and seachem Equilibrium with weekly water changes.
Plants grow well in both tanks and continue to do so, but seem to be doing a little better in the tank with the Seachem Onyx Sand. The tank with the Tahitian Moon Sand developed diatom algae over the same period, but the one with the SOS did not. No big deal. I threw in a Amano Shrimp in the tank with the Tahitian Moon Sand, and he made short work of the diatom algae within a few days. My catfish also develped barbell erosion in the tank with the Tahitian Moon Sand, but I am not sure if the Tahitian Moon Sand caused this as some as that barbell erosion in catfish results from bacterial infections. Plus, that catfish was pushing 9 years, so he may have died regardless.
Tom Barr seems to like Fluorite Black Sand and he has tested many substrates and set up many tanks. If I were confronted with the choices, I would go Fluorite Black Sand.