There appear to be 2 competing theories to explain the origin of the blazer. The earliest on record (1825) is that the scarlet red jackets worn by The Lady Margaret Boat Club (the rowing club of St. James College, Cambridge) were seen to 'set the water ablaze'.

Photo shown with kind permission of 'Fiver' - copyright fiverlocker 2012. We do recommend a visit to Fiver's Flickrstream to view an engagingly quirky collection of images