I spent almost 20 years as a public school teacher, and this kind of alarmist crap was taking up faculty meeting time back when 2 Live Crew was first being banned by WalMart. Here is an example of things to watch for (besides blackness, of course) as reported in the Post:
In student notebooks, they said, scribblings or illustrations often reveal gang affiliations. And synchronized wardrobes can publicly identify gang members or associates. The experts displayed jerseys, bandannas, baseball caps and other paraphernalia confiscated from gang members. There were, for instance, jerseys with the number 13, signifying the gang Mara Salvatrucha, or MS-13.
This type of fear-mongering can also be seen, of course, on the cable trash news channels. MSNBC, in their role as Fox Lite, seems particularly inspired to "inform" the public on the dangers of black males.