To knock out black objects in InDesign, you must prevent the black swatch from overprinting. Unlike most color swatches, which knock out by default, the black swatch overprints by default, including all black strokes, fills, and text characters. The 100% process black appears as [Black] in the Swatches panel. Knock out black objects by either deselecting the overprint default in Preferences or by duplicating the default black swatch and applying the duplicated swatch to color objects that knock out. If you disable the overprint setting in the Preferences dialog box, all instances of Black knock out (remove underlying inks).

It can be cheaper and easier to have the print shop overprint process black on the press.