Blurb gets sRGB JPEGs for printing.
It uses whatever the photo is. If you put a RAW photo into the book, it renders the image you see in the book from the RAW photo. If you put a JPG into the book, it renders the image you see in the book from the JPG.
Whichever the originals are, the book is then created and a file is uploaded to Blurb for printing, and you can't change the internal workings of that part of the program to specify the format of the images.
The only answer to sharpening is to try it and see if the sharpening is acceptable. Everyone is different.
My photos were shot in RAW format. LR made JPEGs when importing.
Lightroom doesn't automatically make JPG files from your RAW files on import.