The filters have usually been super-secret
In case it might be of interest, in the UK, on mobile networks at least, the existence of filters is not (and, as far as I know, has never been) secret, and the categories of content which are likely to render a site being blocked are published too. I appreciate that this is, of course, not the same as a "what's blocked and what's not list".)
The UK's infrastructure mobile operators published the "Code of practice for the self-regulation of new forms of content on mobiles" in January 2004, with the filters being implemented about a year later in early 2005. The code was updated in 2009, and is accessible here. The code still references the Independent Mobile Classification Body, but this is no longer the right place: the IMCB's role has been replaced by the British Board of Film Classification, which also administers the age ratings for films for the UK.
The BBFC documents its approach to mobile content classification on its website, here, including setting out the type of content which the BBFC considers suitable for "adults only", the details of mobile operator contact points in the event that a site operator considers that their site is incorrectly classified, and an appeals procedure against decisions taken by the BBFC.
Whilst there is no published "what's blocked and what's not" list, the mobile operators buy third party services for website classification; most, but not all, buy from Symantec. Symantec has a web interface for its "ratings tool" here, which (after a captcha) lets anyone see how Symantec has classified a particular URL. This is complemented by the Open Rights Group tool (here): the ORG tool does a real-time check of whether a site is blocked across mobile and fixed networks, and the Symantec tool indicates the classification given to the site by Symantec.