A black reflector is mainly used to subtract light rather than add it. In practice, it acts as negative fill: it blocks or reduces stray bounced light and deepens shadows for more contrast and shape.

Useful situations include:

• Portraits where one side of the face is getting too much ambient fill
• Rooms with colored walls, where bounced light adds an unwanted color cast
• Product or still-life shots of shiny subjects, where you want cleaner reflections and more controlled contrast
• Scenes like smoke photography, where reducing spill light helps the subject stand out against a darker background

So although it’s called a reflector, its real purpose is often to absorb light and prevent unwanted bounce, helping you control shadow, contrast, and color contamination.