Save ChromeBlacklistBlocker.ps1 somewhere local.
You can run it via right mouse > "Run with PowerShell".
(this is currently commented out and latest code hides the powershell console window after launch)
Or more permanently, put a shortcut like this into your auto "Startup" folder:
It will monitor the HKLM\Software\Policies registry branch and delete the value named "1" under Google\Chrome\ExtensionInstallBlacklist.
This value is specific to my scenario but is of course editable to your specific needs.
You can test it is working by creating the "1" value yourself and it should disappear.
Another good way to test is to fire gpupdate.exe force a group policy update - again, if you need this, that should make sense :)
More Google search keywords: block registry key