huh! When I hit add page a drop down box gives me photo 1, 2, etc. When I hit any one of the page set ups it gives me pages to add and some choices. I just upgraded Blurb, so the new one will allow 50 pages. What I would do is when you hit the 20 page limit, repeat the add page process until you have all the pages you need for your book.
I have worked with blurb many times. So I know you can have more pages in your book than 20.
BTW. Please also send your question to Blurb's customer service support to see if they can give you a hand. It may take awhile until you can get their reply. But is also worth to try, if you are not in a hurry .