Quick answer to the original question, if anything in a substrate has the ability to buffer the pH at a higher value, it would be limestone. Limestone is white. If you cover it with something to make it look black, it will no longer have buffering capabilities. Ergo, there is no black cichlid sand that buffers the PH up higher.
That said, I like MonkTong's suggestions, and I fully agree with DJRansome that the ability of a substrate to affect pH is overrated. However, the evidence given is not very convincing. Buffering pH means keeping it stable. If you had a tank for five years with aragonite substrate, it started at pH=7.8 and remained at pH=7.8, that could indicate that the substrate buffered the tank perfectly at pH 7.8 :wink: