An ISBN is only required if you were wanting to sell your book on a third-party website (like Amazon) or through a chain bookstore. This is for their inventory management purposes and is necessary only because of the massive volume of books they stock.

If you are wanting people to buy direct from your Blurb profile then no, an ISBN is not required, no matter which format you are selling in. Unfortunately few people buy Blurb books ‘out of the blue’ so be sure to market your publication as much as possible on your own website, Facebook profile, etc.

You could print copies in bulk and see if a local independent bookstore would be interested in stocking it for you. Or ask a cafe about hosting a booksigning and displaying your work on their walls.