I can't recall seeing any black people in the movie, not even among the crowds.
Were there any black actors in V for Vendetta? If not, then why not?
There were a number of black actors in the various flashbacks
As to why there were no black actors in the main cast, the implication (stated rather more clearly in the source comic) was that they were among the "undesirables" who were rounded up during the 'Norsefire purges' and either exterminated or experimented upon to death.
Actually there was a black man in bed with his male lover in the montage of Norsefire cleansings we see as Valerie tells her story. The reason for there are no black people from the POV of the movie present is because England had been ethnically cleansed as this page explains: 'people who were not white, Christian, and heterosexual were rounded up and sent to concentration camps'.