You can cancel your free trial through your Google Play settings in the following way:
Please note: The amount will get blocked (but not charged) on your credit card 24h before the trial expires. If you don’t wish to continue with the subscription, you can still cancel during these 24 hours and you will not be charged. (Further information about Google's authorization can be found here)
1. Open the Play Store app and click on the 3 horizontal bars at the top.
2. Select 'Account'.
3. Click on 'Subscriptions'.
4. Locate the Blinkist subscription & click on 'Cancel'.
5. Confirm the cancellation by selecting 'Cancel subscription'.
6. You can confirm it has been successful as under the subscription it now says the date it will expire, rather than the date of the first charge as seen in step 4 above. Also the 'Cancel' button is no longer be visible.