Why might only some papers from a conference be included in Google Scholar?
Doing a little bit of poking around, I found most articles on BlackHat 2015 not listed in Google Scholar, and all of the articles that I did find listed were cited by other papers.  

It seems to me then likely that this is among the "grey area" sites that Google Scholar is not indexing (BlackHat's status as an academic conference is murky), and thus that any material that you find is being added to the Google Scholar index through other routes.