You can cancel your free trial through your Google Play settings in the following way:
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.