No heat the heater blows cold air but no heat. radiator was flushed not to long ago could it be the thermostat or hoses? or fan doors

Is your air locked slacken off top hose and make sure your getting a good flow of water no air

Check heater core.

thermostat is sticking open

Assuming the cooling system is full and there are no air pockets, are both heater hoses getting hot when the engine warms up ?
If so, it could be a blend door problem under the dash.

Air Temperature Control Blend Door Failure No Heat or no AC (dependes on where door sticks)

In all modern cars there is a tiny DC electric motor driven gear drive that moves a plastic door that blends heated and cooled air from the AC and the heater core, this is called (interestedly enough) the air temperature control blend door actuator. The motor that moves this door fails because the door starts sticking from warping, most common complaint is no heat but it can be no AC as well. To replace these parts (blend door and blend door actuator) you must remove the entire dash from the car. Always have this problem confirmed by a dealer or qualified repair shop as it is a very expensive repair.

Other causes of no heat in the cabin are engine thermostats that are stuck open (engine runs cold) or air bubbles in the cooling system.

If the coolant temp gauge is in it;s usual position when driving(at least 1/3 way up) then flush the heater core with a garden hose.Clamp off the heater hoses,remove them from the heater core and flush back and forth a few times until flow is good,reattach heater hoses and you should have heat as long as the heat/cold airflow door is in the heat position in the heater assembly under the dash.

fix coolant leak where hose or tube goes from overflow jug to the radiator cap and replace radiator or overflow jug pressure cap see what happens.if problem still exist your blend air door stuck or air door vacuum or electrical actuator not working not receiving vacuum could have broken vacuum hose or faulty mode switch, could be many things.could have plugged heater core check both heater core coolant hoses both hoses should be hot.if heater core inlet hose hot and out let hose cold heater core could be clogged need flushing out.

The brown looking chiffon could be oil in the water. However, if you are sure the system is ok then I would check the actuator door motor on the heater and make sure it is allowing the heat into the car. Seems as if you have done as much as you can.

Flapper door inside duct work most likely stuck open not allowing heat to come thru the heater core.

This is a common complaint for the 4.3 v-6 by GM, what happens over time is air gets into the cooling system, usually a small coolant leak at the intake manifold gasket. Ait gets in the cooling system and starts electrolisis. From this the coolant starts to solidify. If I'm correct your heater core is plugged. Remove the radiator cap (engine cold only) and inspect the coolant. The proper color should be red. If the elctolisis is taking place the coolant will turn to rust and look like mudd. If you find mudd in the radiator, most likely the heater core is plugged. This is a very expensive repair. You'll need to remove the raditor & sent it out to be boiled out. Replace the heater core, thermostat and the coolant hoses. You'll also need to perform a "prestone" flush. You may need to flush the cooling system monthly until the system is clean.