A while ago, I attempted a (short) social history of blogs – but limited my references to the past decade. Jim Horton has widened the net in this white paper (pdf file) and provides a perspective from diaries and journalism.
It’s helpful, sensible material. Marketing blogs will mainly fail because – from my PR perspective – marketing seeks to exert control. PR, by contrast, tries to exercise influence. PR practitioners are used to losing control of their messages (though they can’t always admit this to bosses and clients). Blogs may not be a great medium for selling, but they certainly are for arguing and exploring ideas.