In general, cameras operating in black and white mode always capture the RAW image in full color, and then apply a black and white filter to a rendered JPEG version of the image. Because Aperture is expressly designed to leave RAW masters unaltered, it will display the full range of color captured by the camera. If you want to work in black and white, there are several strategies you can use:
The Monochrome Mixer is a good tool that allows you to create black and white versions of your images, with precise control over the red, blue and green levels.
Color Monochrome adjustment allows you to use any color you like to tint your images. For black and white effects, you can use any shade of gray to create the exact tone you're looking for.
The Sepia Tone adjustment, although not strictly speaking black and white, is a good option for creating monochrome versions of your images.
If you wish to use your camera's black and white filter, then shoot in RAW+JPEG mode on the camera, with the desired black and white setting. When you import the images into Aperture, both the full color RAW images and the rendered black and white JPEGs will be added to your library. To work with the black and white images, highlight the thumbnails in the Browser and choose Images > New Version from Master JPEG. A new version of the image will be created next to the original RAW. Note that the thumbnail will appear to be a full color image, but when you click to show it in the Viewer, it will display as black and white.