This is the 1682nd most frequent Russian word.
черном
"Черном" is the prepositional case of "черный," meaning "black" in contexts involving location or association (e.g., "in the black").
The word 'черном' here is used as an adjective in the prepositional case, modifying the noun 'фоне' (background).
На черном фоне белые буквы выглядят ярко.
White letters stand out brightly on a black background.
The word 'черном' is used as an adjective in the prepositional case, modifying the noun 'море' (sea).
В черном море живёт множество различных рыб.
In the Black Sea, many different fish live.
The word 'черном' is an adjective in the prepositional case, used to describe the noun 'коте' (cat).