Depend on maker, black water is actually extract from peat and various organic matter (wood, dead leaf, etc).
Most black water suppose have the same effact as using the peat filtering. It increase the water acidic, add various mineral to the water etc.
Black water element also make the fish to have harder skin therefore make the fish more immune to fungus and parasit.
But is the fish is spawning in tank with overdose black water then the egg shell become too hard for the fry the hatch out n eventually the fry will drown in its own shell.