Quality of Cover Preview
Seems that when the cover image was regenerated by your site, it seemd to artifact.
You can see it here:
How can it be fixed?
The forums are primarily aimed at facilitating BookSmart users exchanging ideas, issues and help, although Blurb staff do monitor them and jump in now and again.
If you have a question of Blurb staff you’d be better asking them directly. Go to Search FAQs and you will see a heading of EMAIL SUPPORT, just choose the appropriate option under that.