I suggest you place your images in InDesign as sRGB (you'll have to go back to the original images, not the ones that are already converted to CMYK). When you've finalised your InDesign document export it Acrobat PDF presets, selecting the Blurb PDF preset (that you've installed). You can view the resultant PDF using Acrobat on your own (presumably calibrated monitor).
When I ask Blurb questions, they take at least one full day to reply... Thank you for being so fast in providing support!
I'll do as you suggest and let you know if it worked!
Thank you again!
I suggest you don't redo the whole document again – just do a couple of test pages first!
1 month of fruitless back and forth discussions with Blurb support (they seem to have no clue...) and then I got my problem solved (apparently) in 5 minutes on Adobe forum...
It seems indeed that using sRGB images in Indesign does the trick... My PDF preview is now matching (as it should) the Blurb preview.
I can't believe Blurb is suggesting to first convert images in CMYK (in photoshop) before placing them in Indesign... All it did was a big mess and an awful waste of time!
Thank you so much Derek!
By the way, what is the explanation for the problem I had? Why does it make a mess if I use CMYK images in Indesign?
PDF export using Blurb settings (using sRGB images in Indesign)
There are more knowledgable people on this stuff on this forum than me [Dov Isaacs], but I think you were converting your colours twice, RGB to CMYK and then again to CMYK, which doesn't help your colour management!
Sounds like a reasonable explanation! If Dov Isaacs or other experts could confirm I would be keen on letting Blurb people know about this because their suggested workflow might really get some other novices like me in trouble and discourage them from using InDesign altogether.
Blurb preview module
I have not printed with Blurb, but If by Blurb preview module you are referring to the Lightroom plugin, that would create a wrong preview because Lightroom has no CMYK or CMYK profile support. See this PS Lightroom help page:
If you are not completely comfortable with color management then Derek's sRGB suggestion is a good one because it delays the conversion to CMYK to output and you are letting Blurb make the conversion, but we would have to assume in the end the conversion would still be to the Blurb CMYK profile.
One thing you have to watchout for is being mislead by out-of-gamut RGB colors. You can work in sRGB but still use the Blurb_ICC_Profile.icc profile for the softproof. In InDesign all you have to do is assign the Blurb profile and turn on Overprint preview.