For that matter what happens when I export to blurb? Is there any behind the scenes upscaling or sharpening? Presumably not.
Robert, this last is probably the most important.
While you have been given info on viewing on screen vs. actual test prints, I am not sure it directly helps you with Blurb.
Testing on a printer will not, most likely, give you any feeling for what output to get from Blurb.
I have not worked with Blurb...directly or via LR. However, in LR, I see no controls for sharpening....and even if you had them, you would still need to get output from Blurbs process to see the results.
It is not clear to me if resizing (up or down) is done in LR before sending to Blurb or is the image, with crop info, is sent to Blurb, with the rest of the layout....and then done there.
It seems like a "black box". Now I do not mind "black boxes" for output....LR resize/sharpening is one, as is Qimage's. However, both have a local (or lab) print which you can view to test/retest until you get it set up for consistent results.
Unfortunately, one does not have that with Blurb. Nor do I see any "levers" which you would have to modify output if you were able to preview (in print) the Blurb output.
John