Most Helpful Girl
There are investigative ways to find out.
First of all, if he sees that suddenly your private messages all say something like, "This user is no longer available" - which happens whether you've deactivated or blocked them - he'll know something is up.
Then he could dig further and check with "someone else", "Hey can you see Xastranger's FB profile because I can't." If the other person says, "Yeah I can see her," then he'll know he's blocked.
If he doesn't want to ask others, he can make a phony FB profile, do a search for you and find that you could be searchable, and then know he's blocked. If you've got your privacy settings on something like you can't be found on a search, there's still further investigating he could do...
On the fake profile, he can go to anyone's profiles that he knows you are friends with on Facebook, and out of all your friends, someone is bound to have their friends' list open for all the public to see. Then - easy search! He just has to scroll through the list and you will be found on his fake, investigative profile, something he wouldn't have been able to see on his normal profile.
So yes, a little digging and it can definitely be discovered that you've blocked. I know because I've done all this, and though I am no longer on FB as an active user, I continue to only keep an active phony FB profile on hand that is totally inconspicuous of who I am for various reasons.