Indigenous cultures can "see" dark constellations (example: the Australian emu in the sky) which are defined empty spaces which are explicitly visible.

Using this concept, one could think of or use blank index cards in a zettelkasten or even the empty (negative) spaces between cards as "dark ideas" (potential ideas which need to be thought of and filled in).

Link to: https://hypothes.is/a/FlqusEN1Ee6XEr_9StPUlA

Dark constellations are dark patches amidst brighter portions of the Milky Way in the night sky which are visible to the naked eye.

Historically they were viewed by Indigenous peoples of Australia as well as Incans.

The emu in the sky is an example from the southern hemisphere. Its 'body' is outlined by Scorpius and Sagittarius and its 'head' is known as as the Coalsack Nebula.

Another example is the Great Rift.