User blog comment:Martialmaniac/Chrollo, that chatterbox/@comment-25657023-20160512011851/@comment-25657023-20160512161438

Well, since Hisoka was in Phantom Troupe before, he might have seen Shalnark using Black Voice at least once, so Chrollo might not have a motive to explain it to him in details, even if it's a limitation that he set for the bookmark, since his opponent already knew the effects of the technique.