When shooting in P, S, A or M modes you can activate the Black & White option by selecting the Monochrome (MC) setting inside the Picture Control menu.
Once you’re inside the Picture Control menu, select the Monochrome (MC) option, then highlight the Toning effect and choose B&W.
This setting will allow you to capture photos in black and white. If you’d like to switch back to color, just access the Picture Control menu again and select the Standard (SD) option.