Is a black dog a bad omen?

yes! although not harmful creatures these animals are looked to as bad omens in some native cultures were they are thought of as ferocious wolf-men.

Another thought

Many Native American spiritual paths revere the wolf as a guardian, and sometimes a messenger from the gods.

Shamanistic shape-shifting is not unheard of and the shaman is rarely an evil force.

Many pagan paths believe the wolf also represents: ritual, loyalty, monogamy and spirit.