We do not block based on searching site contents each time someone asks for it. We block based on a database that was supposed to be reviewed by human beings after being tested with a software. Thus, the probability of blocking a good site is almost negligible. The filter runs after these sites as they get created daily. If a new site has been created now and the searching software hasn't detected it yet, it will not be blocked.
Someone has to be really insisting to bypass the filter.
We filter r-rated sites as well. We believe that r-rated and pornography categories re the same to our culture. R-rated are topless beach photos, for example. We also filter criminal skill sites, e.g. sites that may teach on how to steal or kill. Other categories maybe filtered as well.