Do all Reaction cards completely block all Attack cards?
Reaction Cards:

No. It's not an inherent property of Reaction cards that they protect you from attacks. What makes a card a Reaction is that it's used outside of the normal game sequence when something happens; they don't have to interact with attacks at all.

Moat protects you from attacks because it says it does. Tunnel gives you Gold when discarded, whether or not that is from an attack. Secret Chamber is revealed when an attack is played, but doesn't protect you from it; it just lets you manipulate your deck to attempt to lessen the blow.