In Aperture, the simplest fix is to remove the color entirely: reduce Saturation all the way down, or use the Black & White adjustment. The Black & White tool is usually the better option because it converts using the red, green, and blue channels rather than just muting color, so you can fine-tune the tonal result if needed.

If the blue/purple tint is only a color cast, either method should give you a neutral black-and-white image. You generally won’t need to adjust the channel mix unless the cast is affecting brightness in certain areas.

If the blue stripe is still visible after desaturating, then it’s probably not just color information but an actual density/scan defect. In that case, you’ll need retouching tools beyond a simple B&W conversion—something like Photoshop or another editor with local repair/cloning tools.

So, in short:

1. Try Saturation to zero for the quickest fix.
2. If needed, use Aperture’s Black & White adjustment for better control.
3. If a stripe remains, use a retouching editor to repair it manually.