Heya, so I've noticed this issue for some time now, and am curious if there's a better way to deal with it.
In some cases, when I try to darken a blue sky in a B&W conversion, some parts of the image seem to develop artifacts. One common instance is with leaves and tree branches. The original image does not appear to have any serious CA.
This is working in Lightroom, using the B&W Mix feature with some custom presets to emulate various types of filters. I've tried SilverFX and the results seem to be about the same. The images obviously use progressively stronger red filtration.