Blocks to intimacy are any patterns that we have which are often truly unbeknownst to us. In many cases, we do not even experience them until we enter an adult relationship.
Intimacy requires an enormous amount of vulnerability and trust. For most all of us, the combination of those can bring up a lot of fear. if we have a history of having experiences, where issues around security and trust have been impaired or shamed in some way then that tends to surface as repetitive patterns. These patterns will tend to display themselves when we attempt to get into an adult intimate relationship.
In short, that's what an intimacy block is, they vary but their foundation is often from unresolved issues around trust, trauma or vulnerability from the past.