In Watchmen, towards the end, Dr. Manhattan talks about his joy of being blind to time because of the tachyon emissions, and how exciting it is for him because he had forgotten the joy of not knowing something.
But, we also know that he already knows everything that he will say and do, what will happen, etc. I mean, we can clearly see him seemingly getting fuzzy with the whole "I'm talking to Rorschach 2 minutes ago" thing, and the artwork depicting him doing the exact same thing a page later.
So, presumably he would have been able to see this blind spot from far away time-wise. So, it shouldn't be an actual shock. As we saw with Laurie, who admits to sleeping with Dan, he has to go through the motions of being shocked, because he knows that this is what he will do.
So, did Dr. Manhattan actually not know what was happening the whole time, or was he playacting like always?