The following is taken from the Blurb website:
Our digital printers use the standard four color print process that most every printing press uses: CMYK, which stands for cyan, magenta, yellow, and black.
These four colors are combined to give you the full-color reproduction you expect with printed materials. Monitors and digital images use a different color space called RGB, which stands for red, green, and blue.
The recommended Color Space for Blurb's PDF to Book workflow is CMYK. This will give you the best monitor soft proof representation and the most reliable print results. Use a CMYK color palette for native graphics and text within InDesign® or your layout tool. The digital presses we use are unable to process spot color at this time and that means spot colors will cause problems when files are flattened due to the use of transparency. So, be sure to convert any spot color to CMYK whether in a Graphics Application like Adobe Illustrator® or in a layout tool like Adobe® InDesign®.
• If you prefer to work in the RGB Color Space, we recommend the sRGB Color Space for all of your images. You can soft proof them using the Blurb ICC Profile.
• If you have already converted your images to sRGB you can either keep them sRGB or convert to the Blurb ICC Profile. If your images are already sRGB, it is not necessary to convert to CMYK because the HP Indigo is preset to convert sRGB to CMYK. Just be aware that there will be a slight color shift when this conversion takes place.
• If your images are Adobe RGB, ProPhoto RGB, Colormatch RGB, or another RGB color space, the optimal workflow is to convert your images to the Blurb ICC Profile optimized for the HP Indigo presses that Blurb uses. Color conversion is best done via an imaging program, such as Adobe® Photoshop®, prior to placing your images or graphics into Adobe® InDesign® or your preferred layout tool. Install the Blurb ICC Profile and convert your images to CMYK using the profile.
• If your images are already CMYK, you also should not convert back to RGB or a different CMYK space. If you choose to do so, there is only a small gain in color vibrancy and much more degradation of the file for the amount of effort needed.
So either work in CMYK or sRGB. I am working in sRGB, so according to this, they will be OK.