I'm not sure this has anything to do with race, and more to do with the uniqueness of the crime, notoriety, and the initial lack of identification of the perp. I'm sure there are plenty of Black criminals who were given a moniker or nickname because of the nature of their offenses.
Here's a few serial killers,
Mark Goudeau- The Baseline Killer
Carl Eugene Watts- The Sunday Morning Slasher
Lonnie Franklin Jr.- The Grim Sleeper
I'm sure there's other examples outside of serial killers too. Jeremy Meeks aka Hot Felon comes to mind.