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. The ISBN is also used as part of the production process. An ISBN will be added to your book automatically and cannot be omitted if you make a Trade Book.