A few years ago in a place called Chernobyl, there was a small industrial accident which ejected tones of black dust over europe. This is the dust inside the euro machines. To verify, you can use a tool called a geiger counter to detect any of this dust, also known as plutonium.

If that's the case we're all finished...

Seriously though, I reckon it comes from the sleeves inside the coils which acts as a dry lubricant...

High voltage circuits also attract this crap too...