What does it mean when you dream a black shadow?

There are as many different opinions for interpretation of dreams as there are people that do it. It depends on who you ask.

The mechanism of why we dream is not fully understood, by anyone. That it is a function of the sub-conscious is fairly well agreed upon but there are many theories as to why and how we dream.

People have different solutions with the conscious mind when presented with the same data, it is far more difficult to explain the functioning of the unconscious one.

It is, however, a practical certainty that there is no pre-cognition involved

A different response:

A black shadow is usually regarded as representing an unknown threat. Dreaming about a black shadow is not likely to foresee the future but probably refers to something in the dreamer's present life that feels vaguely uncomfortable or dangerous.