"Black Diamond" is not a brand or specific to one company. It is a term used to describe processed coal slag. Coal slag is the by product of coal used by power utility companies- from the combustion of coal in the generation of electricity. Basically super heated coal. Slag is to coal as ash is to wood.
Here is a bit more technical description:
Coal slag is an black, glass matrix alumina-silicate which is produced when wet bottom boiler power plants burn pulverized coal. The bottom ash is kept in a molten state until it is tapped and introduced to water. The water quenches the molten slag and instantly vitrifies it producing the raw glassy slag which Ensio Resources recycles through its processing facility to produce a variety of beneficial products for market. It has the optimal hardness, density, and particle shape to provide for an exceptional silica free blasting abrasive, roofing granule, non-skid, and sealcoating surface material.
Diamonds are made from coal. Coal is black, therefore the name "Black Diamond".
Unlike clay, it has next to no free silica, and it should be pH neutral.
Here are some various brand names:
ATI Black Diamond
Opta Minerals - Blackblast - Coal Slag
Black Beauty® Abrasive (Boiler slag, Coal slag) from READE
US MIX - Denver, Colorado | BLACK BEAUTY Coal Slag
Black Magnum - Coal Slag, Star Blast, Blastox, Aluminum Oxide ...
Black Magic® Coal Slag - Abrasives Inc
The reason to use this is strictly cosmetic. I do not know of any other practical purpose this would have in the aquarium.