I got a reply from Blurb Tech Support:
Unfortunately, there is no way to import your images from Lightroom into BookSmart. You do have to do it manually. Here are Blurb's recommended guidelines for images used in BookSmart:
1) JPG or PNG file format. (JPG is fine for most images; use PNGs if your images contain text or line art).
2) RGB color (sRGB recommended; no CMYK or Grayscale).
3) 150dpi minimum-300dpi maximum. Out-of-camera JPGs are fine and don't need to be resized unless you're placing many high-resolution JPGs on a single page (for instance, into a yearbook-type layout).
4) Size: images should be smaller than 15MB. The width of vertically oriented images should be less than 4000 pixels. The height should also be less than 4000 pixels. Adding images that exceed those recommended limits may slow BookSmart's performance.