Killah Black was not a concept put forth by mobb deep and the wu tang clan, he was actually a real person. Killah Black was Havoc's older brother who supposedly was a gangster he died in the late 90's in a car crash. Supposedly Havoc could go all over queens without getting robbed or touched when Killah black was alive. The Wu Tang Clan had associated who were rappers called Killa Bees which were there associates.
"U know who killah black is?"
"Yeah isn't he Havoc's gangster brother in QB who died in a car crash"
"Yeah-bless the dead"
killah black may or may not have ever been a real person. Most likely, killah black represents a concept put forth by Mobb Deep and various members of Wu Tang in the early nineties. Killah Black is a name given to a close friend that you would frequently smoke blunts, drink liquor, pull stick ups, and get into trouble with. Killah Black had periodic run-ins with the law, and spent his fair share of nights in the pen. However, if you were to ask Havoc or Prodigy about Killah Black's illegal activities, they would never tell you due to loyalty and love. Killah Black may also be a code name used to avoid saying real names over the phone.
"I'm fallin' and I can't turn back, or maybe it's the words of my man Killah Black, that I can't say, so it's left an untold fact."
-Mobb Deep (Survival of the Fittest)