If its a throwaway character, perhaps not use urban/black/hip hop slang, or if you do, please use it correctly for one. As for physical description, just walk around and see what black people fitting the age, gender, and perhaps the body type of your character(s) are actually wearing in terms of clothing, jewelry, and hairstyles.
Also, I agree with Pantherfan esp. if you are going with a longer-term character. Make them 3-D. However, that might be hard to do if you have a throwaway character. Then again, it might not be. How about you view the interaction first through the villain's eyes. Why are they doing what they do? What is there background? Maybe you can write the encounter through the villain's eyes. Of course you don't have to include it if you don't want in your finished product, but the exercise might help you get inside the head of the villain and make him more real to you as a person and not just a character.
I'm sure that when you read the bios of white, black, brown, etc., criminals there are a lot of things that are similar, so whatever studies you've done into the white criminal mind could also be a resource for you as well. I'm working on my first original novel and I try to put myself in the head's of my characters, I try to find their voice and let them speak through me to some extent. Of course, I have people checking out my work to see if it rings true, esp. when I write female characters.