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Not an easy fix - sounds like either a heater/AC plenum control diaphragm started leaking, or a vacuum hose to one of them has popped off at the actuator or at the heater control, or the vacuum line developed a leak somewhere between an actuator and the heater control or the engine. You can try to pin down by checking airflow out of the defroster, ac and heater vents - if airflow is normal in each position, the problem is probably the heat/cool control actuator, the line from it to the heater control, or the heater control itself.
It sounds like vacuum is not getting to the vent, heat and A/C controls. Find the vacuum can. It may be under the hood on one of the inner fenders. The "one-way" check valve is usually located in the line and close to the can, make sure it works properly. The valve could be the very thing that's causing the problem. Trace the lines back to the vacuum block at the intake manifold or fire-wall, and also back to where the line goes thru the fire-wall. Check the can for leaks or obvious holes. They used to be just a plain metal can, but it could be a plastic ball. Check the lines as you go, for breaks and rub-thru's. Move the controls thru all settings and listen for vent door movement. Worst case is a bad vacuum motor or air flow position selector switch, and/or a faulty Blend Door Actuator. If you have been driving around with the Temperature knob or control set at the same spot for months, move it from fully cold to fully hot several times and wait for the air temperature to change each time. This will help reset the blend door and vacuum control system. Hope this helps... Let me know what you find.
The flap is in the venting itself behind dash. It is not easy to get to. It may be stuck or the vacuum controlled actuator that moves it may be leaky or the vacuum hose that goes from actuator to control switch may be leaky. You can use a vacuum tester to see if it holds pressure.
Inside the vent where the bottom (foot area) vent connects to vent box behind console. There is a flap inside that is controlled by vacuum lines. Either a vacuum line came off, the vacuum actuator is faulty, or the flap is stuck.
There are servos that move air control gates, some were controlled electrically, and some were vacuum. It sounds like there's a vacuum line off the vacuum reservoir under the hood, or a vacuum line has came off one of the servo diaphragms.
This is a bad selector swutch on your Blazer and it requires either a salvage yard part or a dealership part. Don't be surprised if it is a bad solenoid motor that switches the doors inside the dash as well these problems usually run together.
Some heater/defrost controls are mechanically controled and some are vacuum actuated. Look under the passenger side dash and see if you can locate the means by which the flapper is controlled. Normally the flapper just redirects the heated air from the rotating fan to eith go out the vents or up to the vents in the defrost. Most mechanic units (attached by cables from the dash control unit) do not break or wear out. The vacuum operated ones sometimes the small vacuum hoses that attach to a solenoid come off or get brittle and loose their seal against the plastic tips. If you can work the dash controls while you look under the dash and try to see how and why the flapper is not moving, then maybe you can ascertain how the flapper is actuated. Last resort os to try to remove the in dash control unit. It will either have the mechanical cables attached or vacuum lines attached to it so be careful when removing. Once you control the flapper in the air box, then you norally can redirect the air to where it needs to be. Look for a cracked vacuum line by the master cylinder. There are several that interconnect which feed through the firewall and into the dash that supply all of the dash vacuum.