Frank,
I'm not so sure about that. In their Blurb’s PDF to Book Publishing Guide under A Brief Note on Color it says
In the PDF to Book workflow, all RGB color spaces are converted to sRGB, then to CMYK at the HP Indigo print device. Therefore, all CMYK color spaces remain as is and are not con- verted. Depending on your preferred color space, you’ll want to follow these guidelines for optimal print results:
1.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.
2.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 CMYK profile optimized for the HP Indigo presses that Blurb uses. This ICC pro- file – HP5000SemimatteExp05.icc – can be downloaded at blurb.com/downloads/HP5000SemimatteExp05.icc. 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.
This implies you can just use sRGB.
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