Simply start the game and create a black character when asked (when picking a skin color, slide all the way to the right for the darkest). As far as difficulty, some people say to wait till the first fight of the game with the 6th graders, I changed it right away as soon as I got control of my character - to do this, access the Pause Menu via the Options button, go to Combat and choose Mastermind (with the current patch level there is now a level higher than Mastermind available, you do not need to pick it).
Very important: do not touch this setting again for the remainder of the game! Trophy popped for me after the credits (there are two credit rolls, trophy will pop after the second/last credit roll).