Black outline

Unless your style demands it, never use #000000 BLACK color as a outline color. Not only a black line aesthetically boring, black outline is:

1. uninspiring

2. not thoughtful

3. too high contrast when the scene is bright

4. too low contrast when the scene is dark

The function of an outline

1. provide separation between subject from the background

2. provide a hint to the type of material the line is on

3. with line style (on Freestyle), line provides style & the emotional state of the artwork

With black outline, monkey looks harsh and generic

without black outline, monkey looks softer and hinting on the object's material