How does Sapphire put across a collective identity of black british people?

Within the film black people are looked at as being criminals and treated and forced to feel like one. In this case they are not and a white woman who no one believed could be is the criminal. The black people stand together and are always made to look suspicious through their actions and performance for example when John runs away from the police at the club you almost definately think it is him. They are seen to show the same jnterests as white people and also want a relationship with them but the older white people will nto allow this.