I'll give you an example. Say you are running fine but you come to a stop, anywhere, red light in your driveway, parking at work, and when you come to a stop your truck revs high, like 2k or even 2500 rpm. And you just can't seem to find an issue. No codes are popping, And you are driving like normal any other time. This might help. I have 96 GMC sierra. I am putting it here since I also have a 2000 blazer. And my GMC has the 4.3 with a 5 speed, like the blazer. I'll give you a solution and tell you how i came to it after. You might possibly want to check that your upper temp sensor is working fine AND you have a FULL radiator and resevoir of coolant. If the coolant is low enough that it isn't reaching the sensor It could be telling the truck it isn't warmed up and making it rev. Now the fluid doesn't need to be low enough to cause you to over heat but enough to not hit the temp sensor.

Now the background. I got my GMC from my dad 3 years ago when he passed. Turns out he had someone try to do a dexcool conversion to green, (like I suggested, but it failed). Well The truck had all kinds of coolant issues. Thermostat stuck, radiator 1/2 clogged, that damnable quick connect that goes to the heater core was worn out, water pump was worn out. And the upper radiator water hose was mushy. One by one I fixed these. First the radiator, then the pump. When those were fixed found the quick connect whistling. replaced the thermostat (and then the outlet that cracked in half during the repair. And noticed there was still water down around the base of the thermostat inlet. So I knew the upper hose was worn so I tried cutting off the weak end and putting it back on until I got it replaced. (budget). Well truck kept loosing fluid, slowly, And I noticed when my fluid in the reservoir was gone the truck would start to to the reving thing again. It would stop when I had the radiator fluid (green btw) filled but in two days it would start revving again. So I was looking under to see where that fluid could be coming from. That's when I heard the whistle again. But it wasn't the quick connect. It was from the front of the engine. I thought another hose was going bad. NOPE. turns out when I cut the mush off the end of my top radiator hose it of course shortened it. This caused it to rub up against the egr valve plug. Being soft already it wore through in just a week. So last week I replaced the hose. Gave it a few days and voila'. No more reving. And after a week my resevoir is still full. So in short, one easy fix. If you have problems with your truck revving, even revving down and reving back up, at idle (loping) simply check your radiator fluid. if your res is full and your radiator, maybe replace that upper temp sensor. If it's low....BINGO.of course if you fill it and you loose the fluid again and it starts revving again, well start that damnable task of finding your fluid leak (and hope it's not the upper intake gasket. Gotta love the millk chocolate for oil. Been there done that with my old grand am.)