Heater blows cold air, replaced thermostat &

Usually it's a leaky thermostat allowing to much coolant to pass through it when it's closed.To check this, clamp off top rad hose completely(acting like a closed thermostat) If you have hot air from the heater now ,you do need a replacement thermostat.

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.

Sounds like your thermostat is on it's way out. Do a good cooling system flush then replace thermostat.

my 2000 saturn sl blows cold air with the heater on. I changed the thermostat,and checked out my heater core.Still blows cold air.

once truck is warm check both heater core hoses to make sure they are both warm if not then check the cold water valve could be stuck shut not allowing coolant to travel in to heater core.

First you have to identify what causes the problem.
To do this you have to:
1) When engine is cold check the level of coolant. If it is low fill it up.
2) Start engine and check if engine reaches regular temperature. If your engine is not heated enough it means your thermostat does not work properly and should be replaced.
3) If engine temperature is normal, then check the temperature of pipes that go to/from heater core. These pipes are located up the fire wall (easier access is from the passenger side). If pipe/pipes are cold (even engine is worm) it means you heater core is plugged and should be flushed. Keep in mind, flushing the whole cooling system is different from flushing heater core.
4) If pipe/pipes are hot/warm, turn off the fan, switch controller to the hot position, wait for 3-4 minutes and turn on the fan. Note if the cold air blows first or warm air blows first but it cools down in 5-10 seconds. If the air blows cold than it is most probably the problem with blend door or blend door actuator.
5) If air blows warm first and then cools down, then while running engine and remaining the transmission in PARKING position, press on the gas pedal until your engine reaches 3,000 RPM. Check if your heater blows warm/hot air. If it does, then your water pump does not work properly and should be replaced. If the air still cold, I would suggest to flush the heater core.

sounds like the controller went bad use temp checker see what your temp is at inlet of heater core from hose

maybe the heater matrix could be at fault, thats the bit that controls the temperature in the car.

Have the cooling system flushed or remove the hoses from the firewall(when cool) and blow out heater core with compressed air-clogged heater core..

squeeze the hoses to get any air bubbles out and refill coolant as necesary. are you sure you put thermostat in the right way? may be backwards. also may be your sensor.