Immediately after you defeat The Sandman for the first time in the "Mary Jane's Favorite Band" mission and before you "Meet Mary Jane At The Restaurant", you cannot take off the Black suit. During this time period, which does not end until you start the next mission, you can wear the Black suit without any damage being done to you. Use this time to clear the city of all gang activity. This may take awhile, but it is easier with the Black suit.