If you plan to buy or sell your photo book only via Blurb's website, you do not need an ISBN.
If you want to sell your book in a major retailer or through Amazon, an ISBN is required.
If you are making a Trade Book, an ISBN is required. ISBNs are a feature of trade books because these books are optimized for sale in the marketplace. An ISBN will be added to your book automatically and cannot be omitted if you make a Trade Book.