Use Blurb and profiles
When you create a book to print via Blurb in Lightroom, the settings applied to the book are the ones that work best for Blurb. Those settings are 300 ppi, sRGB profile, and standard output sharpening. When Blurb receives the data, they convert the images to CMYK to print to a printing press (similarly to how magazines are printed). CMYK has a smaller color gamut than RGB, so fewer colors can be printed than are displayed on screen.
Blurb does have a CMYK print profile that they send to customers. However, you can't use Lightroom's soft proof feature here either, because Lightroom does not support CMYK color.
Your images are exported to Blurb with the sRGB profile, which limits the number of colors in the image. This profile more closely aligns the way images display with how they’ll print. Viewing the image with the sRGB profile, however, is not always accurate. Never use it as an accurate proof.
If you need exact results, download the appropriate printer profile from Blurb’s site. Use Photoshop to soft proof and print. Or, use the book-creation methods Blurb provides on their site.
You can download the recommended printer profile and get more information from the Color Management Resource Center site at Blurb: http://www.blurb.com/resources/color_management.