Reading a radiator's cold spot, bleeding it safely, and telling trapped air apart from settled sludge.
Trapped air: cold top, warm bottom
Air is lighter than the water around it, so it rises to the highest point inside a radiator and sits across the top, keeping hot water out of that part of the panel. A radiator that stays cold along the top while the bottom is warm almost always has trapped air. Letting it out at the bleed valve lets the radiator fill completely and heat evenly again.
Bleeding safely
Before bleeding a radiator, switch the heating off and give it time to cool: the water behind the valve is hot and under pump pressure while the system runs, so opening it too soon can spray scalding water. Open the valve gently with a key until the hissing stops and water runs steadily, then close it. On a sealed system, check the pressure gauge afterwards, since bleeding loses a little water along with the air and the system may need a small top-up.
Telling air apart from sludge
Trapped air and settled sludge give opposite patterns. Air rises, so it collects at the top of a radiator; sludge is heavier than water, so it sinks and packs into the lower channels instead. A radiator cold along the top is bled; one that is cold along the bottom needs flushing, because bleeding does nothing to shift sludge sitting in the bottom of the panel.
Black water is a corrosion warning, not just air
If the water at the bleed valve runs black and dirty instead of clearing quickly, that is magnetite: iron-oxide debris from corrosion inside the system, usually because there is no effective corrosion inhibitor left in the water. Do not respond by simply bleeding and topping up again and again, because each fresh fill adds more oxygen and feeds further corrosion. The fix is to flush the system clean and restore the inhibitor to its correct concentration.