I had a '79 C-10 "Bonanza" and it is a trim package,as Rene said--if it was factory on your Blazer the dash pad would have "Bonanza" on it near the glove box as well....
I never saw that trim package on a K5,but that doesn't mean GM didn't offer it...more likely the emblems were swapped on at some point,or the fender replaced though..