I do not know if there is a black substrate like this any more.
Soil Master Select, Charcoal is very close to black, but is not available.
Turface used to have a dark one, too, but the only colors I see at their site are gold, red and brown.
Of the colors currently available I like the soft tan and grey blend of STS. Looks very natural, and is reasonably dark, though certainly not black.
I found the KH was very quickly removed. Noticeable in the test results the next day (less than 24 hours) and zero by 48 hours (tap water KH is quite low).
Each time I add baking soda (enough to raise the KH by about 2 German degrees of hardness) it drops to zero in about 48 hours.
I suppose adding a lot of baking soda all at one time might do the trick, but I think I would look for other sources of carbonate, perhaps potassium bicarbonate.
When I add baking soda in small doses I am also doing water changes often enough to get rid of the excess sodium.
If there was any way to keep coral sand, oyster shell grit or limestone sand or fine gravel in larger quantities in the tank or filter, not let it blend with the substrate, I think that would be the best solution. Perhaps a second filter with a lot more of these materials. I am already running 2 filters on the 88 gallon tank, so I could sure put more of something in them, even if I have to reduce the amount of sponges in them.