I have checked ThatPetPlace.com (close by, too) and DrsFosterSmith.com and neither carry Black Flourite. Only "original" and "red." Neither does the Seachem.com web site mention it (as noted above).
Where did the Black Flourite come from? Sounds interesting? Hmm, I bet Tom has an inside track and contact at Seachem.
Tom, you also mention the Alkalinity altering capacity of Onyx Sand. I did not realize it did this. And dangit, I just set up a tank with it. And my tap water is very soft, so I can see that swinging the KH levels all over the place during water changes...
There really is no idea substrate, is there? (I've not tried any of the ADA one's yet, and I cannot find any explanation of the differences between them that give any technical details.)