Wiktionary
black-on-black
a. 1 (context idiomatic English) Something that is invisible or intentionally obfuscated, such as warnings or fine print. 2 A description of the colors of an automobile (e.g., all black rims, paint, and interior). 3 A reference to interactions between black people (e.g., "black-on-black crime").
Usage examples of "black-on-black".
Very soon the largest bunkers became visible, hulking black-on-black out of the stygian darkness.