What does blacking up mean?

blacking up meaning in Urban Dictionary

The work of moving an auto into reverse equipment while on a highway being support and get onto an exit ramp one has already passed away. This term originates from the newest York Metro location, where Asians and Caucasians have carried out a nonscientific research by trying to see what the individual taking this step looked like simply to realize that they've been black colored at least 90% of the time.