heaviness is not a quality of black blades, and haki does not increase the weight of anything, so shusui was heavy even before it became black, that heaviness make it produce larger but slower flying slashes, brook had commented on zoro and ryuma possessing enormous physical strength and destructive capability to be able to project their attacks, also zoro kind of predicted that with the extra weight his slashes will be bigger, another thing is that due to shusui's heaviness the flying slashes it produces are bigger thus absorbing the smaller slashes of the other swords, which benefited zoro the most with his nitoryou and santoriyou, ryuma was ittoryou user.