saw one like that not long ago. can't recall if it was this same unit or not. it may have been. if there are any orignally built with those black top pickups, it is very, very, few. So possibly/likely a few black pre-date the whites, which pre-date the chromes. or possibly a few blacks are simply randomly distributed, as Guild bought and used whatever was available.
some may have simply been painted black. and perhaps that is the more likely scenario? hard to say.