I'd suggest you check out
www.coloradok5.com if you want to see multiple examples of roof racks on a Blazer. Also, you might check the forums on
www.expeditionportal.com. Both those guys get real serious about building roof racks on their rigs.
And don't worry too much about the structural integrity of the K5 fiberglass roof, they are much stronger and more durable than most folks give them credit for. I've owned a 78 GMC Jimmy for over 30 years that has had both a roof rack as well as a spare tire carrier mounted to the removable roof, and there have been zero consequences with over 200K miles of bad Northern Rockies roads. Check out the pictures - no cracks ever. There is a slight bow in the roof now, but no structural damage, and believe me that tire was heavy.