Blog Link Network (Black Hat Link Selling Scheme)
Blog networks are sometimes abused by black hat search engine optimization providers in order to give a false appearance of authority towards specific websites, which gives them a higher ranking in Google. In March 2012, Google took unprecedented action against this unethical behavior, by removing hundreds of websites from their search index.
Blog networks and content networks are often considered to be the same thing by internet marketing beginners and SEO newbies. They are not the same thing, since 'black hat' blog networks are created primarily to support link schemes and sell page rank to a group of "members" or "buyers" while Content Networks can provide authentic and legitimate "sticky" content that attracts both Google Page Rank (because the content is nice to link to) as well as human traffic (because interesting content and blog posts attract interested visitors.
Content Networks have been sold for tens of millions of dollars in liquidity events. An example of a company who has purchased complete, in-tact content networks is Demand Media.
For clarity, Please Also See Content Networks