bootstrap - bg-dark vs navbar-dark

You also need to add the class bg-dark additionally to navbar-dark.

navbar-dark basically just determines what color the text should be i.e. light text for a dark-background navbar and dark text for a light-colored navbar.

bg-dark then determines the actual background-color of the navbar. But other colors would work just as well. For example, bg-primary or bg-danger would work well with navbar-dark.