Darker Overall Printing?

I have 12 books and all are photo essay style books. I have noticed this shift to darker printing that some other authors have mentioned in the last blogs as well. My books from a year ago were pretty much WYSIWYG. Now, its a crapshoot but everything tends to be darker. While I don’t mind the saturation in color, you certainly tend to lose highlights in some shots. Is there a Blurb person who can respond to this? Has some overall change in printing or machine setup caused this?