Hi Phil
Is it all over the carpet? are you sure that these are not rogue tufts?
this can easily trick you into thinking that its a spot stain when in fact it will be a dark piece of yarn thats wrapped itself around the tuft.
No amount of cleaning will remove it, it will have to be pulled and re-tufted.
If it is fibres from socks etc that are adhering to the tips then the carpet,to prevent this happening again, will need an anti-static treatment
