How much Online Content in Blended Learning?
In 2007 The Sloan Consortium ( presently the Online Learning Consortium) asserted that when 30-79% of class content is available online that is a blended learning class.
entire report here: http://www.onlinelearningsurvey.com/reports/blending-in.pdf
Choice within Online Content
Another often referenced definer of Blended Learning is The Christensen Institute. Student control of Time, Place, and Path are important in this definition.
more on blended learning in this IMS blog:
more on online learning in this IMS blog