I shoot a lot of landscapes and frequently darken the sky by lower the blue Luminance slider. I always get a "fringe' aloing the edge of the hills - see example below. I usually darken the sky after I convert to B&W but it doesn't seem to matter, I still get the fringe. Here is an example: http://www.davidpalermo.com/data/web/fringe.jpg
On the right image, all I did was slide the blue and aqua Luminance sliders to darken... As you can see there is a nasty white outline fringe on the hill's edge where it meets the sky. I did lens correct and CA correct and I still get this fringe. Is this a bug?
1) What version of LR (i.e. 3.6, 4.1 Final Release), 2) what Blue Luminance setting are you using, 3) and what do you see without Blue Luminace and any of the lens corrections and CA corrections applied?
1. LR 4.1 final. 2. It's the only blue luminance I am aware of. It's under Color / HSL / B&W.
If I don't apply lens correction I see the CA fringe. You can try this too... open a landscape with a blue sky and crank the blue luminance slider down. It looks as if there is always a light outline around the mountains or other objects such as birds against the sky. I will compare in Silver Effex 2 but I don't think there is a way to turn down luminance for a single color so it may not work.
Here is another example. This is a picture of a large rock at the ocean... as you can see the fringe is quite pronounced! This rock is very sharp so I guess it's not a soft focus issue. http://www.davidpalermo.com/fringe2.jpg
What type of fringing is this? Is it the fringing that the Auto-CA checkbox attempts to remove and that gets worse further from the center of the photo or is this axial fringing that occurs in front and in back of the focus plane and needs to be removed with the manual-CA fringing ranges and amount sliders?
In LR4.x the Auto and Manual CA-fringe removal is independent of the profile-based lens-corrections. When you say you disable lens-corrections the color-fringe appears, do you mean you are setting the profile to none, or turning off the entire Lens Corrections section in LR?
Maybe you could link to an example RAW with the problem, so we can try it, ourselves. Upload to something like www.dropbox.com or www.yousendit.com then post the public download link.
I was able to duplicate what you are seeing, but it took a lot of searching. It appears when using a lens with moderate to high CA (red/cyan fringing), in a high-contrast situation, with a very high (-100) HSL Luminance setting. I could only duplicate your results using an image shot with a Canon 18-55mm II kit lens with a 600D.
Crank your image view up to 2:1 and then disable/enable 'Remove Chromatic Chromatic Aberration.' It appears to be an interaction between the HSL mask and CA removal image mask. Lacking any further experimentation my best suggestion would be to keep your HSL settings to no more than -50.
I noticed you're using a Canon 16-35mm L Series lens. I'm just wondering if you may have a defective lens. I couldn't find any images shot with my comprable Canon17-40mm lens on a 5D MKII that were anywhere near that bad. Can you post the same 1:1 crop with no CA removal and no HSL sliders, but in Color mode? Also what camera are you using with the 16-35mm?