It seems Killerbee only charged one of his swords the moment before he blocked, so Sasuke wouldn't have seen it before hand, because it wasn't charged, before hand. That and what everyone else is saying, he was surprised because he thought he would have bested Bee with his chidori sword, but was bested instead.
That was at a different moment, but agreed, he also got bested instead when he thought his sharingan would best Bee
You would have thought Sasuke would have stopped underestimating Bee after he 'died' the first time, but no Sasuke is stubborn and adamant that he is better than all. Such a joke.
Sig By Damon Max
I agree, but following that thinking, Sasuke lost. Bee got away without being harmed, while Sasuke was left licking his wounds and literally died how many times during that battle?