You can cancel your free trial through your iTunes settings in the following way:
1. Open the iTunes app and scroll down to click on your Apple ID.
2. Select 'View Apple ID'.
3. Scroll down through the options to reach the Subscriptions button.
4. Click on 'Subscriptions'.
5. Locate your Blinkist subscription on click on it.
6. Select 'Cancel trial'.
7. Click on 'Confirm' to confirm the cancellation.
8. You know it has been successful as the 'Cancel trial' button has disappeared.