Do you have any update on this errand? Did Blurb respond and what did they say?
I just ordered a book and experience the same issue with a white line at the bottom corner of every spread that gets thinner and thinner till it disappears at the top corner. My workflow was 1) design the book in InDesign, 2) Export to PDF in InDesign, 3) Check the the PDF-file in Adobe DC and 4) Upload the PDF to Blurb and send to shipping. Everything looked and still looks fine in Adobe DC (step 3), so I can only assume something went wrong when it was uploaded or printed.
Like you I also ordered Priority shipping to get it as fast as possible, but could that really have something to do with it?