The University of Manitoba's Learning Technologies Centre has assembled a brief literature review on blended learning formats/definitions. This brief compilation surfaces the complexities inherent in blended learning approaches
The LTC server at the University of Manitoba has been decommissioned and so this link no longer works. In my (ultimately unsuccessful) search for this lit review, I came across this 2011 COHERE report on blended learning: http://cohere.ca/wp-content/uploads/2011/11/REPORT-ON-BLENDED-LEARNING-FINAL1.pdf; there's a decent reference list at the end of the document. COHERE is a Canadian group, Collaboration for Online Higher Education and Research.