I'll jump out there with a big guess, I was hoping someone who knew would respond. I've read here that some of the earlier round eye Blazers and Burbs had heat/AC fans that ran at a low speed, even with the fan switch all the way down - there is no OFF position. Someone else here even said it's like that so you won't get exhaust in the passenger area, that it's there to create positive air pressure inside the truck so that any exhaust fumes in the low-pressure area at the back of the truck won't be drawn inside the cab around any air leaks that might be present in the tail gate area.
Someone else here, member motornut who has two round eye K5's, posted in a "favorite budget mod" thread Here:
Your Favorite Budget MODs ($50) that he likes to snip the low wire on the switch so the fan will have on off position. See post 14. John (motornut) does not say what color wire it is, and he does not post near as much as he used to but he does still log on pretty regular. If someone here doesn't respond what wire it is, might send John a message. He won't bite.
HTH