How to report app developer to Apple?

You will have an email invoice from Apple for the purchase. That email contains a "Report a Problem" link. But you can only report problems within 90 days of purchase.

iTunes Support - Get Support

Apple provide the site https://getsupport.apple.com as the starting point for App Store support. From this link you can navigate through:

• iPad
• App Store, iTunes Store & Music
• The topic is not listed

Keep in mind, your purchase and relationship is with Apple, and not the developer of 1Blocker. Apple handle the money and your relationship. The developer has no specific details of you or your purchase.