Blurb's Trade book and magazine formats are only available in portrait orientation, with the binding on the long edge. We can't produce these formats in landscape orientation, with binding on the short edge.
However, if you're able to produce a print-ready PDF, you might be interested in our sister site, MagCloud. With MagCloud you can produce a Standard Landscape publication or a Digest Landscape publication. You'll find templates for InDesign and other design programs via those links which will allow you to create PDF files for uploading to MagCloud.