Thanks for the suggestion, but I'm still having trouble. Double sided layout does make it easier to make distinctions between left and right pages, but I still don't know where to set bleed space. While editing a masterpage, I can go to Page > Manage Page Properties... to set margins, but I don't see anything about bleeds.
Meanwhile, the other part of my original post still stands. If anyone reading this has any experiences using Scribus to publish a book with Blurb, please let me know about your experience. Any bits of advice would be greatly appreciated!