One thing that doesn't make much sense to me in this game is that enemies that are "sleeping" (either as a result of Assassination's blackjack attack or because they were randomly generated that way) can counter/block/dodge attacks. The same thing goes for monsters who haven't noticed your presence. Thus, I suggest a small buff to both Blackjack and sneaking in general: Enemies who are "asleep" or unaware can't dodge/block/counter your attacks, and the first hit against them (the one that wakes them up/makes them aware of your presence) is always critical.
Hitting from behind, if you afford it, should yield a critical hit. Be it by stunning, sleeping or spawning asleep.
I think its better that they do attack and dodge and such. There is a psionic spell that puts them to sleep. If u gained attack bonus when theyre asleep it was be way too overpowered.