The plugin window can be completely blank for three reasons:
Your Revit is not up to date
We always recommend keeping your Revit updated with the latest updates made available by Autodesk, these updates are always made to correct several errors in Revit and one of these errors may be affecting the plugin's operation. For example, we know that in Revit 2020 until version 2020.2.5 there was an error that prevented the plugin from working, and only in version 2020.2.6 this error was solved by Autodesk.
The plugin cannot access the internet
The plugin only works with internet access, and if something prevents access, it will not work and the screen will be completely blank.
There are several factors in Windows that can prevent the plugin from accessing the internet, there may be some blockage in the Windows firewall settings that prevents Revit from accessing the internet, and consequently Plugin Blocks, or your anti-virus may be causing this lock.
To understand and troubleshoot the plugin's connection to the internet, we recommend that you seek technical support for Windows.
Your Plugin is not up to date
If the window appears blank with the message "not found", it means that your Plugin version is outdated.
The plugin normally updates automatically on your computer, and only very old subscribers need to worry about updating it. Versions prior to 1.0.3 do not have the automatic update functionality, these will need to be uninstalled and downloaded again from the Blocks website.