Black men in white movies usually have one of these qualities. If not all.
Hen pecked. Scared of their over baring wife who screams the whole scene. Maybe even looks at the lead and asks, "Who does this white boy think he is?" (Martin Lawrence in "Wild Hogs.")
Crazy hair. Afros, long curls. Not a normal hair cut. Something that makes them stand out more than just being the only black person in the group.
Not a threat to your women. They are usually the one gay character, they cross dress at some point or they are married to some crazy woman. This way they are not going to have a chance to beat out the white lead. (The Black guy in Greek, the black guy in the last two seasons of "Boston Legal.")
He next time you look at a commercial with a group of people - 20 somethings hanging out or office drones in the break room talking about lunch - check out the one black guy. Not two.
Ok, we've done it. Let's move on.