I can shut everything off but I still get a small amount of heat coming through the heater. It's great in the winter, but the temps are 90 degrees plus. Any suggestions? year 2000 4WD

 

Vacuum leak or the blend door is malfunctioning. I'd lean towards a vacuum leak.
If you can hear a hissing in the cab &/or the vehicle runs kinda rough, crappy mileage, hissing under the hood - vacuum leak.
Check all the tubes from the engine to the brake booster, to the ball under the hood or box in the DS fender well (don't know what you're driving & that would help).
On my 2002 the box in the fender well (GM saving space idea grrrr) is a pita to get to with the extra plate they use to mount it inside the fender well above the fender well lining. But they do rot out & crack & a vacuum leak on our rides can cause major malfunctions.

 

+1, does it go away if you use the floor vent?

 

I thought that was normal operation at least in some cars and the only way for it to go away is to switch it to inside/recycled air from outside/fresh air however I'm not sure if blazers have that option.

 

They all do this. You can't completely shut the air off in these vehicles. You'll always have some air coming through the floor vents, and the air flow will vary by how fast you're driving.

 

yep...my 2000 ls does this also.