Those great people at Blurb just announced PDF to Book! This is a fantastic development. You can now use inDesign with custom templates from Blurb to create your book offline in inDesign then simply export as PDF and upload.
This opens up some great possibilities, you really could now create just about any book you desired and you get the great benefit of working in inDesign. Can't wait to try this. You can read more about this cool development here. Templates are all avaliable for download already along with detailed resources for getting started.