Some people do block others. Some people just have a lot of walls around them and aren't open. Most people I've known who are like that have a tough outer shell but actually are quite brittle inside. It tends to be a defensive mechanism from what I've experienced, though some people enjoy holding their cards close to their chest as it were. They may even accuse you of 'not giving much of yourself'...

I could be wrong though.

Other people have a difficult time trying to get a read on me, but for me the 'blocking' is usually deliberate. I'm naturally fairly reserved in person and like to keep the upper hand with strangers if I can. But again, it's still a protective mechanism; I have no interest in conning or manipulating people for my own uses and suck at "networking" for just that reason.