Thank you for reaching out.I understand that you have added custom domains for your Azure Front Door and backend is a static website that you have hosted on Blob Storage. The requirement here is to make backend host URL not accessible from Internet.
The recommended solution here will be to use Azure FrontDoor Premium tier and access Azure Storage account by using Private Link.
You can go through this article to understand how this set-up works.
In this set-up the storage account is configured to deny direct access from the internet, and to only allow requests through the private endpoint connection used by Front Door. This configuration ensures that every request gets processed by Front Door, and avoids exposing the contents of your storage account directly to the internet.
You can follow the tutorial here to configure Azure Front Door Premium to a storage static website with Private Link.
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