You develop a blog, and sell it. Developing meaning install a blog on a domain, adding a theme, maybe adding articles, etc.
if the blog is good and the niche is good then you can make somewhere around $200 - $1000 per blog. If you blog grows up big you can expect more..
Johns Wu recently flipped Bankaholic.com for $15,000,000 after two years of developing it. If you work hard, the sky is the limit.