I received a slightly delayed response to my question to Blurb regarding lack of quality on my second blurb book. They kindly offered a coupon for $10 off my next book, and then explained that 300 dpi was the best to use:
We recommend 300 dpi. 180 dpi should still print well, but 300 is better. You will get a warning symbol if you place a picture in a container that is under 150dpi. … DPI is about the pixels per inch – the more pixels the better. For example, say you want a full bleed page (10×8 book), then the image needs to be 2888 x 2475 pixels at 300 DPI. This is derived by the formula of 9.625-inches x 300 dots per inch = 2888 pixels in the horizontal; and, 8.250-inches x 300 dots per inch = 2475 pixels.
I wasn’t able to find a recommended DPI on their website; perhaps I missed it.
It does however, makes sense to me, even though the MyPublisher response was essentially “180 is the most we can do.” So… I shall print a blurb book one more time, and (again) hope for the best. As I’ve noted, their books sizes are better than Apples (i.e. more then one) and their prices are superior as well (20-40 pages for one price).