Unfortunately, iPhone's Messages app doesn't allow you to view blocked messages on iPhone. Once you block a number, iOS doesn't allow the blocked number's SMS and iMessage texts to get delivered to your device. Since these messages are not stored in a hidden folder or blocked inbox, they won't show up in your conversations. The sender is still able to send messages, but they will be silently filtered out. You have to unblock the contact from Settings > Messages > Blocked Contacts in order to get new messages from them. Also, you won't be able to view or recover messages sent while the number was blocked.