A black cat whose fur appears brownish in the sunlight is technically piceous, or blackish brown. Piceous is most commonly used in science to describe the glossy, dark shells of certain insects.
Piceous is primarily found in biological and geological descriptions. Derived from the Latin word for "pitch," it describes a deep, brownish black, usually with a slight sheen. You would likely use it to describe the dark hue of mineral-rich soil or the obsidian-like shell of a beetle, rather than everyday items like chocolate cake or hair color. It is a precise term for when something isn't quiet "true black," but instead has a warm, dark undertone.