Henning Hraban Ramm
Perhaps describe a bit more exactly what you'd like to achieve. As far as I understood: You want a black rectangle at the margin that has no text in it, always same size, but moving position per chapter?
Yes. More precisely, chapter 1 - 5 would have a black box in the margin at the top right of each right page, then chapters 6 to whatever have the next box below, and so on. I usually do this ‘manually’ in a DTP app (by defining a set of master pages and applying them selectively to a group of ‘chapters’) but ConTeXt is clearly capable of doing it automatically. And I bet ConTeXt could also be persuaded to place white text on the box as well... I’ve read through the MetaFun manual. It’s amazing technology, but a bit too advanced for me at present.