InDesign black plate separation issues

When you're working in InDesign it is important to remember that it automatically sets black to overprint. That is the '[Black]' swatch that comes preinstalled on every new document. For the most part this is of little consequence. But occasionally, for example when doing a spot metallic ink that has to overprint, no matter how hard you try your plates won't work out.

These two crests are in an InDesign document and are meant to print as a black shape with overprinting gold foil filigree.  They are stacked as follows… A large outer gold shape, a smaller black shape over this and overprinting gold filigree on top of everything.

As you can see they appear identical onscreen, however the one on the top uses InDesign's '[Black]' swatch whilst the other uses a newly created swatch which is just 100% black.

When you view the separations you get two very different results. They both generate the same, correct black plate, a solid black shape.

But two very different gold plates…

Because InDesign sets its '[Black]' swatch to automatically overprint EVERYTHING the black shape on the top didn't knock out the gold underneath resulting in a solid gold shape with no filigree. But the one below which was coloured with the new 100% black swatch DID knock out and everything separated as expected.

Note the new colour swatch does not generate a new black plate, the way using Pantone Black would for example.