Technically speaking 403 can be returned if it's blocked for legal reasons, but obviously it would be nice to know why it is forbidden.
Still, imho, 4XX could be a response for a given URL, but when "a website is blocked" (from the text of the previous URL), we should go to 5XX, as in 503 - service unavailable.
Consider for example the blocking of sites in the UK, or the blocking of the PirateBay website in the Netherlands. Those are all very limited audiences where the location of the client is causing a legal reason why the content can not be displayed.