Final answer:
The term 'black shadow' is used for descriptive imagery, often to suggest something ominous or intensify the darkness of the shadow.
Step-by-step explanation:
The phrase black shadow typically serves to enhance the descriptive imagery within a literary context. Its purpose is often to evoke certain emotions or to paint a vivid picture in the reader's mind. Shadows are inherently without color, as they are an absence of light, but the addition of the adjective 'black' emphasizes the depth or darkness of the shadow, possibly suggesting something ominous or foreboding.