It sure seemed at first that the damage type on this was hitting an opponent's biggest weakness, but after a while I started to get some results that didn't seem to fit. It might just be RNG, or perhaps it might be a weighted chance and it's more likely but not guaranteed to hit an enemy's weak point.
Edit: Possibly it's just random as the first technique in a chain, but otherwise follows the damage type of the technique before it.
Also appears to be nothing special following Dragonblood with the stone equipped, just the result for following Stealth.