I haven't seen actual bagged black aragonite. Caribsea has a mixed black and white live sand - I think the white is aragonite. Another company sells Calcite with the brand name 'Gray Coast Calcite' you can buy at Petco.

I tried black silica sand in one of my old nanos - the shed from the live rock and such made it look ugly in short order - you would have to vaccuum it weekly to keep it black. In a larger tank if you got finer grained aragonite you could get a few cukes , some ceriths and a conch or two to keep it nice and clean.