What does it mean to see a black shadow rat?

It,s time to call in the exterminators.

Rat is usually a sign of treachery, deceit, fear, a nagging doubt, or loss. I would take it as a warning, of the possibility of up coming danger and be prepared. Fore warned is fore armed, so keep your eyes and ears open. You may experience a theft or deliberate attack and if you are ready for it can avoid physical harm or great loss. Do not do anything foolish, things can be replaced, life can't.