Firstly, I assume you've read over this article regarding content-filtering which may assist with some of your questions around URL string Whitelisting/Blacklisting. - https://documentation.meraki.com/MX-Z/Content_Filtering_and_Threat_Protection/Content_Filtering. If you're blocking all categories to try and find out what a particular blocked site is apart of, Meraki have already publically stated that they use "Brightcloud" for URL categorization, you can find a category for a particular site by using this lookup tool- https://www.brightcloud.com/tools/url-ip-lookup.php Are you running the MX will full list or top sites only?
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