It’s almost certainly due to the monitor being too bright and not calibrated to match print output. I’ve made that expensive mistake myself in the past and it’s really disappointing. I always recommend with any printer (if not using your own) doing some test prints at various brightness and contrast settings so that you can be confident going forward.
Monitor calibration also needs doing on a regular basis after the panel has warmed up as changes will take place over time during usage.
No, images are not underexposed in LR. I adjusted each photo of course. Perhaps as the following reply indicated the issue is color calibration of my iMac monitor to the printer used by blurb. LR outsources the Book to them for printing. I have no way to know their printer calibrations. Oh well, lesson learned.
I have had several books printed by Blurb (Australia)- all have been magnificent. Totally happy with quality, colour and contrast.
My monitor is calibrated with a Spyder. I send my book to Blurb using the Book module in LR.
(Never a thought as to profiles or printer calibrations!)