There's a few ways to do black & white adjustments. Here are three that may be helpful. They're all essentially the same, but with slightly different control over how the image or individual colours within it are adjusted.
Black & White Adjustment
Aperture has a
Black & White adjustment for very simple B&W conversion. It lets you choose how the Red, Green and Blue channels are combined for luminance (typically ~30%, 59% and 11% for R, G and B respectively). This is likely to be the simplest method. You shouldn't need to adjust the sliders, unless the blue/purple tints are significantly affecting the luminance (e.g. if they're 'added' to the image, rather than just a slight bias in Blue & Red channels).
B&W Adjustment, default settings.
There's also a
Color adjustment that lets you modify Hue, Saturation & Luminance of up to 6 individual colours (with a specified range as to how close to said colour the adjustment takes place), so you could also specifically just desaturate your blue and/or purple tints. It may help to use the 'dropper' tool (in the circle on the left) to pick the right blue/purple colour tint for adjustment. I've found this tool useful for adjusting just one or two colours in the image, without affecting others. If your blue stripe is noticeably brighter, for example, you could also adjust its luminance (to a negative value), so as to hide it completely. If the purple cast is even, then it shouldn't need anything other than desaturation.
Color Adjustment, default Blue color fully desaturated & reduced brightness.
Or as @dpollitt points out, you can also just desaturate the image with the saturation slider. I believe this is the same as using the default settings on the
Black & White adjustment (30%R, 59%G, 11%B).
Enhance Adjustment, whole image fully desaturated.