Shadowblade obscurity is a weakness rather than a strength. No assassin can be everywhere, but an assassin like Deathmaster Snitch is so stepped in the lore of his unstoppableness that he serves the council of thirteen simply by existing. The knowledge that he always lurks in the shadows to do the bidding of clan Eshin prevents more skaven from turning traitor than he could ever possibly kill. As to which of them would win in a fight, I think the point is moot. Neither of them would dream of engaging in open combat and would only strike against such a dangerous foe when entirely sure that he had the element of surprise. Given that the very existence of the Shadowblade is unknown to most, finding his location is nigh impossible. Similarly, there are few surface-things that know of the existence of skavenblight, much less the location of a single, almost magical gifted skaven in the sprawling under empire. As neither of them can locate the other to gain the necessary advantage of surprise, neither would ever engage the other.
Despite their infinite wisdom and intricate knowledge of magic, the old ones failed to foresee the purple sun of Xereus.