Magnetite?

Hi. I have a 1960s house with a combi boiler fitted in 2009. Heating working well except for one rad which won't heat adequately. Patchy heat - along the top, some at one side. No air coming out when I bleed it. Am thinking sludge.
I thought I'd loosen the hex blank at the other end to see what happened. Got it all the way off. No water or air.
Stuck my finger into the hole. Came out black and oily.
Is this a case of magnetite and if so, is it necessary to flush the whole system or could a single rad flush and some inhibitor afterwards do the trick?
With many thanks in advance.

Presumably the blanking plug was on the top of the radiator?
If it was black up there then certainly there could be magnetite issues. Consider turning the radiator off at both valves and then disconnecting them from the radiator (sludgy water will run out so prepare for this), then get the radiator into the garden for a wash through with a hose pipe.
Now - turn each valve on to see if water is flowing (you'll lose boiler pressure here so top it up again).....if you get water at the valves then let it run clear. At least you can assume you can get circulation at this point.
Reconnect the radiator, turn it on, bleed and repressurise as necessary......lets know how it goes!
(At this point, any inhibitor in the system will be diluted, and that won't help in the future.....you may need a power flush of the whole system too.)
John

Thanks. The blanking plug is at the top. Hoping it doesn't come to a full system flush. Fingers crossed it's just the one. Pity we just had new carpet fitted Many thanks for your help

When that black shyte gets on her brand-new carpet you will find that nothing takes it off.
So you need to do lots of covering prep work before you disconnect any pipework.
One technique to limit spillage is to gently loosen the connections then try and lift the radiator off its brackets and, whilst still connected, rotate it down to the horizontal position. Disconnect and turn radiator bottom up so bottom connections are now at the high point.

Great advice. Will let you know how we go on.
Many thanks!

Normally you can lift fitted carpet by pulling it up from the grippers. Just enough so you can get some old towels down on the floor.

Even turning it upside down won't stop all the spillage, dress the room up like a crime scene and have a bucket ready to tip the rad into, also get your hand over the opening on the rad.
Worth a try to drain the whole system to reduce the amount of water in that radiator. You can also get plastic things to catch the water and direct it into a tray.
But the main defence is plastic sheets with loads of towels on top