In a review on a repository that has restricted access we disable revision links:

Unfortunately, we don't disable the "Blame" button. Instead, we display an error message when it's used: "An error has occurred. No permission to access repository 'foo'".

Additionally, we provide 4 links that lead to 403 pages: "Download Raw File", "View File History", "View File Source", "View File Diff".

Some claim we should not even display the file contents at all, but that's a separate discussion (CRUC-5638).