A block page can only be injected with HTTP - it won't work with HTTPS sites.
The continuing proliferation of HTTPS (now at ~95%, as observed by Google) is one of the major reasons why Pi-hole has switched to NULL blocking (unspecified IP blocking) as default in 2018, and why the blocking page finally has vanished from Pi-hole in 2022.
If you'd wanted to have an HTML block page nevertheless, you'd have to figure that by yourself.
Upcoming Pi-hole changes has some basic suggestions under its Changes to the default “Block” Page (404 handler) section.