Allows you to specify which extensions the users can NOT install. Extensions already installed will be disabled if blacklisted, without a way for the user to enable them. Once an extension disabled due to the blacklist is removed from it, it will automatically get re-enabled.
A blacklist value of '*' means all extensions are blacklisted unless they are explicitly listed in the whitelist.
If this policy is left not set the user can install any extension in Google Chrome.