Unfortunately, you cannot check block messages on iPhone for either SMS or iMessage because Apple completely prevents blocked messages from being delivered. Once a number is blocked, any text from that person is not only prevented from appearing in your Messages app, but also that message is not stored in a blocked folder anywhere. This holds for both regular SMS and iMessage chats. The only exception to this is blocked voicemails, which might be found in Phone > Voicemail > Blocked Messages. If you want to receive new text messages, simply navigate to Settings > Messages > Blocked Contacts and remove the number from the block list. It's important to know that messages sent during the blocking period are irrevocably filtered and can't be recovered later.