I have used three different types of black substrate, and the black blasting sand is my favorite (and the cheapest).
The first one I tried (and the one I am still using in my 25 gallon tank) is the black Activ-Flora from Nature's Ocean. It looked good at first, and still looks okay, but the substrate has many sized particles in it, and over time the small stuff settled on the bottom, so now it looks like I have black pebbles, which isn't exactly the look I want.
Opinions of my 25g (9/18 rescape)
Here is an old thread with pics of this tank and substrate. You can see the gravelly look it has now. Right under the small piece of driftwood, towards the middle of the first pic, you can see the smaller size particles where one of my fish moved all the large pieces.
The second one I tried was the Flourite black sand. I had it in my 16 gallon bowfront. It looked awful. The sand was actually more of a gray color than a true black. It was also way to easy to stir up. The tank always looked dirty and the floating sand ruined my HOB filter. I have no pics of this tank. It never looked good. (The only reason I kept it up for so long was because I had a pair of pelvicachromis taeniatus "kienke" in there that bred like mad.)
The black blasting sand was the third black substrate that I have used. I am currently using it in my 40 breeder, and I also used it in a 29 before I took it down. Not only does it look better, being a true black, but the plants loved it. The roots from the plants I pulled out of that tank (the 29) were unbelievable. I don't have any pictures from then, and the 40 breeder I have now has cichlids in it, so there are no plants in the substrate. I really wish I would have found out about this before spending more money on (in my opinion) an inferior product.
Comments/suggestions for my 40 breeder cichlid tank
This thread shows some bad pics of my 40 breeder how it looks right now.