Dave coughed up some electrons that declared:
It doesn't work like that.
The blue sheet is almost waxy, but just good enough to receive the toner and survive the fusion bar. There's no ink besides the toner.
So the blue sheet comes out with an image of the tracks just adhering to it.
When pressed to a copper board with a hot iron, the toner melts and finds it get's on better with the copper so it sticks to that instead and releases from the blue sheet.
The toner, being basically plastic gunk resists the etchant well and thus a PCB is born.
Cheers
Tim